Seikan Tunnel（Ja:青函トンネル）is a railway tunnel in Japan. It connects Hokkaido to Aomori Prefecture. This is the longest tunnel in the world. The tunnel is 53.85 km (33.49 miles) long. 23.3 km (14.5 mile) of the tunnel is underwater. There are two stations in a tunnel.
These stations are used in case of emergency.