An immersed tube is a kind of underwater tunnel composed of segments. It is constructed off site (prefabricated) and floated to the tunnel site to sunk into place and then linked together. It is commonly used for road and rail crossings of rivers, estuaries and sea channels/harbours. It is often used in conjunction with other forms of tunnels at the ends because they are necessary to continue the tunnel from near the water’s edge to the entrance at the land surface. There are accounts of this method being used for pedestrian tunnels underneath the Thames but not for tube tunnels.