Strait, northwestern Pacific Ocean. It extends from the Sea of Japan (East Sea) to the open ocean between the islands of Honshu and Hokkaido, Japan. It is 15–25 mi (24–40 km) wide. The Seikan Tunnel linking Aomori and Hakodate cities runs beneath the strait. Completed in 1988, it is the longest railway tunnel in the world; 14.5 mi (23.3 km) of its 33.5 mi (53.9 km) length lie under the strait.
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Japan applies the 3-nautical-mile (5.6 km) territorial limit to the Tsugaru Strait and allows foreign ships to pass through the Strait.
The Tsugaru Strait has eastern and western necks, both approximately 20 km across with maximum depths of 200 and 140 m respectively.
In the past, the most common way for passengers and freight to cross the strait was on ferries, approximately a four-hour journey. Now the Seikan Tunnel provides a convenient but more expensive alternative and approximately halves the travel time in comparison to ferrying. When Shinkansen trains can traverse the tunnel to Hakodate (scheduled for 2015), the journey time will be cut to 50 minutes.
In 1954, 1,155 lives were lost in the strait, on the freightliner Toya Maru.
Thomas Blakiston, an English explorer and naturalist, noticed that animals in Hokkaidō were related to northern Asian species, whereas those on Honshū to the south were related to those from southern Asia. The Tsugaru Strait was therefore established as a major zoogeographical boundary, and became known as the "Blakiston Line".