Saikan Tunnel.

The 53.85 km long Saiken tunnel links the islands of Honshu and Hokkaido. It still holds the record for being the longest undersea tunnel. The Shinkansen running through the tunnel are limited to 120 kmh (they can do 320 kmh) as freight trains also use the tunnel and run the risk of being derailed if the Shinkansen ran at full speed.