Seto Inland Sea

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Seto Inland Sea
Seto Inland Sea.jpg
View of the Seto Inland Sea from Miyajima island
Japan natural location map with side map of the Ryukyu Islands.jpg
Red pog.svg
Seto Inland Sea
The Inland Sea and its major straits
Location Pacific Ocean
Coordinates 34°10′N133°20′E / 34.167°N 133.333°E / 34.167; 133.333 Coordinates: 34°10′N133°20′E / 34.167°N 133.333°E / 34.167; 133.333
Type Sea
Basin  countries Japan
Surface area23,203 km2 (9,000 sq mi)
Average depth38 m (125 ft)

The Seto Inland Sea (瀬戸内海, Seto Naikai), sometimes shortened to the Inland Sea, is the body of water separating Honshū, Shikoku, and Kyūshū, three of the four main islands of Japan. It serves as a waterway connecting the Pacific Ocean to the Sea of Japan. It connects to Osaka Bay and provides a sea transport link to industrial centers in the Kansai region, including Osaka and Kobe. Before the construction of the San'yō Main Line, it was the main transportation link between Kansai and Kyūshū.


Yamaguchi, Hiroshima, Okayama, Hyōgo, Osaka, Wakayama, Kagawa, Ehime, Tokushima, Fukuoka, and Ōita prefectures all have coastlines on the Seto Inland Sea; the cities of Hiroshima, Iwakuni, Takamatsu, and Matsuyama are also located on it.

The Setouchi region encompasses the sea and surrounding coastal areas. The region is known for its moderate climate, with a stable year-round temperature and relatively low rainfall levels. The sea is famous for its periodic red tides (赤潮, akashio) caused by dense groupings of certain phytoplankton that result in the death of large numbers of fish. Since the 1980s, the sea's northern and southern shores have been connected by the three routes of the Honshū–Shikoku Bridge Project, including the Great Seto Bridge, which serves both railroad and automobile traffic.


The International Hydrographic Organization's definition of the limits of the Seto Inland Sea (published in 1953) is as follows: [1]

On the West. The southeastern limit of the Japan Sea [In Shimonoseki-kaikyo. A line running from Nagoya Saki (130°49'E) in Kyûsû through the islands of Uma Sima and Muture Simia (33°58',5N) to Murasaki Hana (34°01'N) in Honsyû ].

On the East (Kii Suidô). A line running from Takura Saki (34°16'N) in Honsyû to Oishi Hana in the island of Awazi, through this island to Sio Saki (34°11'N) and on to Oiso Saki in Sikoku.

On the South (Bungo Suidô). A line joining Sada Misaki (33°20'N) in Sikoku and Seki Saki in Kyûsyû.

The range of the Seto Inland Sea by the Territorial Sea Law (領海及び接続水域に関する法律) is 19,700 km2 (7,600 sq mi). The range of the Seto Inland Sea according to the Setouchi Law and the Setouchi Law Enforcement Order is 21,827 km2 (8,427 sq mi).

Geographical features

The Seto Inland Sea with Shikoku and Chugoku from the ISS Chugoku-Region-Shikoku-Japan-ISS-Space.png
The Seto Inland Sea with Shikoku and Chūgoku from the ISS
The beauty of the islands of the Seto Inland Sea, Suo-Oshima, Yamaguchi Prefecture Setouchi.jpg
The beauty of the islands of the Seto Inland Sea, Suo-Oshima, Yamaguchi Prefecture

The Seto Inland Sea is 450 km (280 mi) long from east to west. The width from south to north varies from 15 to 55 km (10 to 34 mi). In most places, the water is relatively shallow. The average depth is 38 m (125 ft); the greatest depth is 105 m (344 ft). [2]

Hydrologically, Seto Inland Sea is not a true inland sea, being neither an epeiric body of water like Hudson Bay nor an isolated endorheic basin like the Caspian Sea. Rather, it is actually a marginal sea; a division of a wider ocean (in this case the Pacific) which is partially enclosed by islands, archipelagos, or peninsulas, (here, the Japanese Home Islands) adjacent to or widely open to the open ocean at the surface. The Naruto Strait connects the eastern part of the Seto Inland Sea to the Kii Channel, which in turn connects to the Pacific. The western part of the Seto Inland Sea connects to the Sea of Japan through the Kanmon Straits and to the Pacific through the Bungo Channel.

Each part of the Seto Inland Sea has a separate name in Japanese. For example, Iyo-nada (伊予灘) refers to the strait between Ehime, Yamaguchi, and Ōita prefectures in the western portion of the sea; Aki-nada (安芸灘) is the open expanse west of the Geiyo Islands, near Hiroshima prefecture; and Suō-nada (周防灘) refers to the expanse between Yamaguchi prefecture and Suō-Ōshima. There are also many straits located between the major islands, as well as a number of smaller ones that pass between islands or connect the Seto Inland Sea to other seas or the Pacific. Almost 3,000 islands are located in the Seto Inland Sea. The largest island is Awaji-shima, and the second largest is Shōdo-shima. Many of the smaller islands are uninhabited.

Major islands


Over 500 marine species are known to live in the Seto Inland Sea. Examples are the ayu, an amphidromous fish, horseshoe crab, finless porpoise, and great white shark, which has occasionally attacked people in the Seto Inland Sea. In the past, whales entered the sea to feed or breed, however because of whaling and pollution, they are rarely seen.


Seto Inland Sea seen from the Torii of Itsukushima Shrine ItsukushimaTorii7492.jpg
Seto Inland Sea seen from the Torii of Itsukushima Shrine

During the last ice age the sea level was lower than today. After the ice age, sea water poured into a basin between the Chūgoku mountains and Shikoku mountains and formed the Seto Inland Sea as we know it today. From ancient times, the Seto Inland Sea served as a main transport line between its coastal areas, including what is today the Kansai region and Kyūshū. It was also a main transport line between Japan and other countries, including Korea and China. Even after the creation of major highways such as the Nankaidō and San'yōdō, the Seto Inland Sea remained a major transport route. There are records that some foreign emissaries from China and Korea sailed on the Seto Inland Sea.

The importance of water traffic gave rise to private navies in the region. In many documents, these navies were called suigun (水軍, "water army"), or simply pirates. Sometimes they were considered to be public enemies, but in most cases they were granted the right to self-governance as a result of their strength. During the feudal period, suigun seized power in most coastal areas. The Kono in Iyo Province (today Ehime Prefecture) and Kobayakawa (later Mōri) in Aki Province (today a part of Hiroshima Prefecture) clans were two of the more famous suigun lords. In the 12th century, Taira no Kiyomori planned to move the capital from Kyoto to the coastal village of Fukuhara (today Kobe) to promote trade between Japan and the Song dynasty of China. This transfer was unsuccessful, and soon after Kyoto became the capital again. Later, the Battle of Yashima took place off the coast of present-day Takamatsu.

In the Edo period, the Seto Inland Sea was one of the busiest transport lines in Japan. It was a part of a navigational route around Japan's islands via the Sea of Japan. Many ships navigated from its coastal areas to the area along the Sea of Japan. Major ports in the Edo period were Osaka, Sakai, Shimotsui, Ushimado, and Tomonoura. The Seto Inland Sea also served many daimyōs in the western area of Japan as their route to and from Edo, to fulfill their obligations under sankin-kōtai . Many used ships from Osaka. Thanks to transport through the Seto Inland Sea, Osaka became the economic center of Japan. Each han had an office called Ozakayashiki in Osaka. These Ozakayashiki were among Japan's earliest forms of banks, facilitating domestic trade and helping to organize the income of the daimyo, which was in the form of koku , giant bales of rice.

The Seto Inland Sea was also part of the official Chosendentsushi route, bringing Korean emissaries to the shogunate. After the Meiji Restoration, the coastal cities along the Seto Inland Sea were rapidly industrialized. One of the headquarters of the Japanese Navy was built in the town of Kure. Since the Meiji period, development of land transport has been reducing the importance of the Seto Inland Sea as a transport line. Remarkable land transportation innovations include the San'yō Main Railroad Line in Honshū and the Yosan Main Railroad Line in Shikoku (both completed before World War II) and three series of bridges connecting Honshū and Shikoku (completed in the late 20th century). The Seto Inland Sea is still used, however, by an international cargo transport line and several local transport lines connecting Honshū with Shikoku and Kyūshū.


Major cities with heavy industrial activity on the coast of the Seto Inland Sea include Osaka, Kobe, and Hiroshima. Smaller scale manufacturing and industry can also be found in Kurashiki, Kure, Fukuyama, and Ube in Honshū, and Sakaide, Imabari, and Niihama in Shikoku. Major industries include steel production, vehicle manufacture, ship building, textiles, and since the 1960s, oil refining and chemical products. Imabari Shipbuilding, Japan's largest ship building company, has its headquarters and some of its yards in Imabari, Ehime Prefecture. Thanks to the moderate climate and beautiful landscape, fishing, agriculture, and tourism bring a lot of income to the area as well.


Major highways in the Seto Inland Sea. Yellow: Kobe-Awaji-Naruto. Green: Seto-Ohashi. Red: Nishiseto Expressway. Map of Honshu-Shikoku Bridge Project.svg
Major highways in the Seto Inland Sea. Yellow: Kobe-Awaji-Naruto. Green: Seto-Ōhashi. Red: Nishiseto Expressway.
KURE-MATSUYAMA ferry, Seto Inland Sea 2017 Wake behind a ferry.jpg
KURE-MATSUYAMA ferry, Seto Inland Sea 2017

Today the Seto Inland Sea serves its coastal areas mainly for two purposes: first, international or domestic cargo transportation, and second, local transportation between coastal areas and islands on the sea. Major ports are Kobe, Okayama, Takamatsu, Tokushima, Matsuyama, and Hiroshima.

Historically, the Seto Inland Sea as transport line served four coastal areas: Kansai, Chūgoku, Shikoku, and eastern Kyūshū. The Seto Inland Sea provided each of these regions with local transportation and connected each region to the others and far areas, including the coastal area of the Sea of Japan, Korea, and China. After Kobe port was founded in 1868 to serve foreign ships, the Seto Inland Sea became a major international waterway with connection to the Pacific.

Development of land transportation shifted the travel between east and west that is, between Honshū and Kyūshū to railroad and road transport. Two coastal railways, San'yō Main Line in Honshū and Yosan Main Line, were built. Those railway lines stimulated the local economy and once invoked a rail mania. Many short railroads were planned to connect a certain station of those two lines and a local seaport on the Seto Inland Sea, and some of them were actually built. The Ministry of Railroads, later the Japanese National Railways and then Shikoku Railway Company, ran some train ferry lines between Honshū and Shikoku including the line between Uno Station (Tamano) and Takamatsu Station (Takamatsu). When the Great Seto Bridge was finished and began to serve the two coastal areas, that ferry line was abolished.

Highways & bridges

Akashi Kaikyo Bridge Akashi Bridge.JPG
Akashi Kaikyō Bridge

The main islands Honshū and Shikoku are connected by three series of bridges since the late 1980s. This improves land transportation between the connected islands. These series of bridges, collectively known as the Honshū–Shikoku Bridge Project, are, from east to west, Akashi Kaikyō Bridge, Great Seto Bridge, and Nishiseto Expressway.

Kobe-Awaji-Naruto Highway

The easternmost highway was built between 1976 and 1998. It leads from Akashi (Hyogo prefecture) on the Akashi Kaikyō Bridge (the longest suspension bridge in the world) to Awaji Island, from there via the Ōnaruto Bridge to Ōge-jima (Naruto, Tokushima Prefecture) beyond the 1.3-kilometer wide Naruto Strait and finally across the Muya Bridge to Shikoku.

Seto Chuo Highway

The Great Seto Bridge connects Okayama Prefecture with Kagawa Prefecture since 1988. It consists of a total of six two-storey bridges, whose lower floors are used by the railway (Japan Railways Group). The high speed Shinkansen does not go to or on Shikoku.

Nishiseto Highway / Shimanami Highway

This is the first of three intersections of the Seto Inland Sea. Construction started in 1975, but was fully completed in 1999. It connects the Nishiseto- Onomichi Highway in Hiroshima Prefecture with a total of ten bridges and several smaller islands with Imabari in Ehime Prefecture. Approximately 100,000 people live on those islands. The bridges are: Shin Onomichi Bridge, Innoshima Bridge, Ikuchi Bridge, Tatara Bridge, Ōmishima Bridge, the two Ōshima bridges and the three Kurushima Kaikyo bridges. The Kurushima-Kaikyō Bridge connects the island of Ōshima to the main island of Shikoku.

Major tourist sites

Mount Ishizuchi Mount Ishizuchi.jpg
Mount Ishizuchi
Mimosusogawa Park (ja:mimosusoChuan Gong Yuan ) in Shimonoseki Mimosusodouzou.JPG
Mimosusogawa Park (ja:みもすそ川公園) in Shimonoseki

The coastal area of the Seto Inland Sea is one of the most famous tourist destinations in Japan. Even before Japan opened to foreigners in the middle of the 19th century, the sea's beauty was praised and introduced to the Western world by those who visited Japan, including Philipp Franz von Siebold, and after the country's opening, Ferdinand von Richthofen and Thomas Cook.

Its coastal area, except for Osaka Prefecture and a part of Wakayama Prefecture, was appointed the Setonaikai National Park (瀬戸内海国立公園, Setonaikai Kokuritsu kōen) on March 16, 1934, as one of three oldest national parks in Japan.

Itsukushima Shrine, on the island of Itsukushima in the city of Hatsukaichi, is a UNESCO World Heritage Site and one of the most famous Japanese sites outside Tokyo and Kyoto. Shōdoshima, nicknamed the "island of olives", and the Naruto whirlpools are two other well-known tourist sites. Neighboring locations like Kotohira and Okayama are often combined with the tour of the Setouchi region. Some historic sites, including Yashima in Takamatsu and Kurashiki, also attract many visitors. Hiroshima is the neighbor city to Itsukushima Shrine and another UNESCO World Heritage Site because of atomic bomb damage in 1945. Idol Unit STU48 operate on a cruise on the Setuchi.

The eastern end of the Sea is now famous for the Setouchi Triennale set up in 2010 with the next event happening in 2019. Some of this takes place on the island of Naoshima, known colloquially as the art island, and the home of several permanent museums.

At the far eastern extremity, as the Sea meets the Pacific Ocean, are the Naruto whirlpools that can be reached by sight-seeing boats.

The Shiwaku Islands are a defined group numbering 28 that can be reached by ferry boat from Marugame. Here Richard Henry Brunton built one of his lighthouses that can still be seen, and the grave of Frank Toovey Lake, a young midshipman in his survey party has become famous.

In the central area of Seto Inland Sea is Mount Ishizuchi on Shikoku. It is the highest mountain in western Japan and the highest mountain in Shikoku.

In the western end of the Sea is Mimosusogawa Park (ja:みもすそ川公園) in Shimonoseki. It commemorates the final stage of the Genpei war between the feudal Taira clan and Minamoto clan (1180–1185).


Some sites along the Seto Inland Sea were featured in eighth-century Japanese literature, both in prose and in verse, including Kojiki , Nihon Shoki , and Man'yōshū . Since some sites were used as places of exile, their feeling and landscape were evoked in waka. In fiction, in The Tale of Genji , Genji fled from Kyoto and resided in Suma (now a part of Kobe) and Akashi for two years.

In medieval literature, because of the Genpei War, the Seto Inland Sea is one of the important backgrounds of The Tale of the Heike , particularly in its latter part.

In the Western world, Donald Richie wrote a literary nonfiction travelogue called The Inland Sea relating a journey along the sea, beginning from the East at Himeji and ending at Miyajima in the West, close to Hiroshima, going from island to island, exploring the landscape, meeting and discussing with local people, as well as musing on Japanese culture, the nature of travel and of identity, and his own personal sense of identity. In 1991, filmmakers Lucille Carra and Brian Cotnoir produced a film version of Richie's book, which further explored the region through interviews and images photographed by Hiro Narita. Produced by Travelfilm Company and adapted by Carra, the film won numerous awards, including Best Documentary at the Hawaii International Film Festival (1991) and the Earthwatch Film Award. It screened at the Sundance Film Festival in 1992. [3]

Koushun Takami's novel Battle Royale took place on a fictional island in the Seto Inland Sea.

A critical plot element of the Japanese series Fafner in the Azure is an alien life form discovered at the bottom of this sea known as the Seto Inland Sea Mir.

Related Research Articles

Honshu Largest island of Japan

Honshu, historically called Hondo, is the largest and most populous main island of Japan. It is located south of Hokkaidō across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyūshū across the Kanmon Straits. The island separates the Sea of Japan, which lies to its north and west, from the North Pacific Ocean to the south and east. It is the 7th largest island in the world, and the 2nd most populous after the Indonesian island of Java.

Shikoku Island and region of Japan

Shikoku is one of the five main islands of Japan. Shikoku is the second-smallest main island after Okinawa. It is 225 km or 139.8 mi long and between 50 and 150 km or 31.1 and 93.2 mi wide. It has a population of 3.8 million. It is south of Honshu and northeast of Kyushu. Shikoku's ancient names include Iyo-no-futana-shima (伊予之二名島), Iyo-shima (伊予島), and Futana-shima (二名島), and its current name refers to the four former provinces that made up the island: Awa, Tosa, Sanuki, and Iyo.

Ehime Prefecture Prefecture of Japan

Ehime Prefecture is a prefecture of Japan located on the island of Shikoku. Ehime Prefecture has a population of 1,342,011 and has a geographic area of 5,676 km². Ehime Prefecture borders Kagawa Prefecture to the northeast, Tokushima Prefecture to the east, and Kōchi Prefecture to the southeast.

Okayama Prefecture Prefecture of Japan

Okayama Prefecture is a prefecture of Japan located in the Chūgoku region of Honshu. Okayama Prefecture has a population of 1,906,464 and has a geographic area of 7,114 km². Okayama Prefecture borders Tottori Prefecture to the north, Hyōgo Prefecture to the east, and Hiroshima Prefecture to the west.

Tokushima Prefecture Prefecture of Japan

Tokushima Prefecture is a prefecture of Japan located on the island of Shikoku. Tokushima Prefecture has a population of 728,633 and has a geographic area of 4,146 km². Tokushima Prefecture borders Kagawa Prefecture to the north, Ehime Prefecture to the west, and Kōchi Prefecture to the southwest.

Kagawa Prefecture Prefecture of Japan

Kagawa Prefecture is a prefecture of Japan located on the island of Shikoku. Kagawa Prefecture has a population of 949,358 and is the smallest prefecture by geographic area at 1,877 square kilometres (725 sq mi). Kagawa Prefecture borders Ehime Prefecture to the southwest and Tokushima Prefecture to the south.

Awaji Island

Awaji Island is an island in Hyōgo Prefecture, Japan, in the eastern part of the Seto Inland Sea between the islands of Honshū and Shikoku. The island has an area of 592.17 square kilometres. It is the largest island of the Seto Inland Sea.

Honshū–Shikoku Bridge Project System of bridges

The Honshu-Shikoku Bridge Project is a system of bridges connecting the islands of Honshu and Shikoku across the Inland Sea of Japan, which were previously only connected by ferry. It consists of three major connections. All bridges are now controlled by the Honshu-Shikoku Bridge Expressway Company and the Japan Expressway Holding and Debt Repayment Agency (日本高速道路保有・債務返済機構). The system consists of three expressways and their respective bridge systems.

Nishiseto Expressway

The Nishiseto Expressway, often called the Shimanami Kaidō (しまなみ海道) is an expressway in Japan that connects Onomichi, Hiroshima and Imabari, Ehime, going through nine of the Geiyo Islands, including Ōshima, Ōmishima, and Innoshima. The road and multiple bridge crossing across the Seto Inland Sea is one of the three main transportation links of the Honshū–Shikoku Bridge Project constructed between the islands of Honshu and Shikoku.

Ōnaruto Bridge

The Ōnaruto Bridge is a suspension bridge on the Kobe-Awaji-Naruto Expressway connecting Minamiawaji, Hyogo on Awaji Island with Naruto, Tokushima on Ōge Island, Japan. Completed in 1985, it has a main span of 876 metres (2,874 ft). Although it is one of the largest bridges in the world, it is dwarfed by the Akashi-Kaikyo Bridge, which is on the same route. In 2004, 6.8 million cars and trucks crossed this bridge, translating into a daily average of about 18,600.

Setonaikai National Park

Setonaikai National Park is a national park comprising areas of Japan's Inland Sea and of ten bordering prefectures. Designated a national park in 1934, it has since been expanded several times. It contains about 3,000 islands, known as the Setouchi Islands, including the well-known Itsukushima. As the park is formed of many non-contiguous areas and covers a tiny proportion of the Inland Sea's total extent, control and protection is problematic, with much of the wider area heavily industrialized.

Yosan Line Railway line in Japan

The Yosan Line is the principal railway line on the island of Shikoku in Japan, connecting the major cities of Shikoku, and via the Honshi-Bisan Line, with Honshu. It is operated by the Shikoku Railway Company, and is aligned approximately parallel with the Inland Sea coast, connecting the prefectural capitals of Takamatsu and Matsuyama and continuing on to Uwajima. The name of the line comes from Iyo () and Sanuki (), the old names of Ehime and Kagawa, respectively.

Takamatsu, Kagawa Core city in Shikoku, Japan

Takamatsu is a city located in central Kagawa Prefecture on the island of Shikoku in Japan, and is the capital city of the prefectural government. It is designated a core city by the Japanese Government. It is a port city located on the Seto Inland Sea, and is the closest port to Honshu from Shikoku island. For this reason, it flourished under the daimyōs as a castle town in the fiefdom of Takamatsu, during the Edo period. Takamatsu is a city with a large concentration of nationwide companies' branch offices, which play a large role in its economy, and it contains most of the national government's branch offices for Shikoku. The castle tower formerly used as the symbol of the city was destroyed during the Meiji period. In 2004, construction of the Symbol Tower, the new symbol of Takamatsu, was completed. The Symbol Tower is located in the Sunport area of the city. The Symbol Tower is the tallest building in Takamatsu, and is right next to another tall building The JR Clement Hotel, which is also part of the Sunport complex.

Imabari, Ehime City in Shikoku, Japan

Imabari is a city in Ehime Prefecture, Japan. It is the second largest city in Ehime Prefecture.

Ōshima (Ehime)

Ōshima (大島) is an island in the Geiyo Islands, lying between the larger islands of Honshū and Shikoku in the Seto Inland Sea in Japan. It is part of Ehime Prefecture and is governed by the city of Imabari.

Japan Median Tectonic Line

Japan Median Tectonic Line, also Median Tectonic Line (MTL), is Japan's longest fault system. The MTL begins near Ibaraki Prefecture, where it connects with the Itoigawa-Shizuoka Tectonic Line (ISTL) and the Fossa Magna. It runs parallel to Japan's volcanic arc, passing through central Honshū to near Nagoya, through Mikawa Bay, then through the Inland Sea from the Kii Channel and Naruto Strait to Shikoku along the Sadamisaki Peninsula and the Bungo Channel and Hōyo Strait to Kyūshū.

Honshu-Shikoku Bridge Expressway Company

The Honshu-Shikoku Bridge Expressway Company Limited, abbreviated as JB本四高速 in Japanese or HSBE in English, operates the Kobe-Awaji-Naruto, Nishiseto, and Seto-Chūō expressways and their respective bridges between the islands of Honshu and Shikoku, Japan. It is headquartered in Chūō-ku, Kōbe, Hyōgo Prefecture.

Akashi Strait Waterway between the Japanese islands of Honshu and Awaji

The Akashi Strait is a strait between the Japanese islands of Honshu and Awaji. The strait connects Seto Inland Sea and Osaka Bay. The width of the Akashi Strait is approximately 4 kilometers, and maximum depth is about 110 meters. The fastest tidal current is about 4.5 metres per second.

Setouchi region Region

The Setouchi region, or simply Setouchi, is a geographic region of Japan. Setouchi includes the Seto Inland Sea and the adjacent coastal areas of Honshū, Shikoku, and Kyūshū, three of the four main islands of Japan.

Kobe-Awaji-Naruto Expressway

The Kobe-Awaji-Naruto Expressway is a tolled expressway that connects Hyōgo and Tokushima prefectures in Japan by crossings of the Akashi Strait and Naruto Strait. Built between 1970 and 1998, it is one of the three routes of the Honshu-Shikoku Bridge Expressway Company connecting Honshū and Shikoku islands. The route is signed E28 under Ministry of Land, Infrastructure, Transport and Tourism's "2016 Proposal for Realization of Expressway Numbering."


  1. "Limits of Oceans and Seas, 3rd edition" (PDF). International Hydrographic Organization. 1953. Retrieved 28 December 2020.
  2. "Inland Sea". Encyclopædia Britannica Online. Retrieved 22 February 2013.
  3. NY Times review