How Undersea Tunnels Are Built

Two methods dominate. Bored tunnels, like the Seikan Tunnel and the Channel Tunnel, are driven through rock beneath the seabed with giant machines or drills.

Immersed tunnels, like the Marmaray and the coming Fehmarn Belt link, are built as concrete or steel sections in a basin, floated to site, sunk into a dredged trench and sealed. Bored tunnels suit deep rock, immersed tubes suit soft sediments and shipping channels..