The 53.85 km (33.46 mi) Seikan Tunnel opened between the cities of Hakodate and Aomori, Japan.

The Seikan Tunnel is a 53.85-kilometre (33.5-mile) dual-gauge railway tunnel in Japan, with a 23.3-kilometre (14.5-mile) segment running beneath the seabed of the Tsugaru Strait, which separates Aomori Prefecture on Honshu, Japan's main island, from the northern island of Hokkaido. The tunnel's track level lies approximately 100 metres (330 ft) below the seabed and 240 metres (790 ft) below sea level. Following several decades of planning and construction, the tunnel opened on 13 March 1988.

Hakodate is a city and port located in Oshima Subprefecture, Hokkaido, Japan. It is the capital city of Oshima Subprefecture. As of January 31, 2024, the city had an estimated population of 239,813 with 138,807 households, and a population density of 354 inhabitants per square kilometre (920/sq mi). The total area is 677.86 square kilometres (261.72 sq mi). The city is the third largest in Hokkaido after Sapporo and Asahikawa.

Aomori , officially Aomori City, is the capital city of Aomori Prefecture, in the Tōhoku region of Japan. As of 1 August 2023, the city had an estimated population of 264,945 in 136,781 households, and a population density of 321 people per square kilometer spread over the city's total area of 824.61 km² (318.38 sq mi). Aomori is one of Japan's 62 core cities and the core of the Aomori metropolitan area.