apan 500 Yen Coin, Seikan Tunnel Opening, 1988. Obverse: Perspective view of the Seiko Tunnel, surrounded by waves and sea gulfs. Reverse: Map of the isles of Japan connected by the Tunnel, within band. The Seikan Tunnel ) is a 53.85-kilometre (33.46 mi) railway tunnel in Japan, with a 23.3-kilometre (14.5 mi) long portion under the seabed. Track level is about 140 metres (460 ft) below seabed and 240 m (790 ft) below sea level.