The Transbay Tube is an underwater rail tunnel that carries Bay Area Rapid Transit's four transbay lines under San Francisco Bay between the cities of San Francisco and Oakland in California. The tube is 3.6 miles (5.8 km) long, and attaches to twin bored tunnels. The underground section of rail between the nearest stations (one of which is underground) totals 6 miles (10 km) in length. The tube has a maximum depth of 135 feet (41 m) below sea level.