Immersed tube
![Immersed tube type A, built upon a shallow trench on the seabed.](/Images/godic/202502/18/Immersed_tube_type_a.svg2256.png")
![Immersed tube type B, built under the seabed level.](/Images/godic/202502/18/Immersed_tube_type_b.svg2256.png")
![Construction of segments for the second Coen Tunnel in the Netherlands](/Images/godic/202502/18/2e-Countunnel-in-Aanbouw2256.jpg")
![Tunnel segments sealed off, ready to be floated to site, and sunk into place](/Images/godic/202502/18/2011-01-31_Tweede_Coentunnel_Bouwdok_Barendrecht2256.jpg")
An immersed tube is a kind of underwater tunnel composed of segments, constructed elsewhere and floated to the tunnel site to be sunk into place and then linked together. They are commonly used for road and rail crossings of rivers, estuaries and sea channels/harbours. Immersed tubes are often used in conjunction with other forms of tunnel at their end, such as a cut and cover or bored tunnel, which is usually necessary to continue the tunnel from near the water's edge to the entrance (portal) at the land surface.