text: There are several ways to build tunnels underwater.\n\nOne way is to build sections of tubes in advance, float them into position using tugboats, sink them, have divers go down and seal the sections together, and then pump the water out.\n\nA more traditional way is to dig a tunnel far enough below the sea bottom that the rock over them keeps out the water.  It's kind of like digging a mine,\n\nIf the rock or soil isn't able to keep out all the water while the tunnel is being dug, sometimes it's possible to pump compressed air into the tunnel, so the pressure inside the tunnel is greater than the water pressure outside, and have workers seal the inside of the tunnel with a concrete or steel lining.