"In engineering, a caisson is a retaining, watertight structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships."

In this case, the Caisson No 2 is situated alongside the Sarsia Fisheries Research Vessel in the East Float Dock in Birkenhead. It is also the sister structure of the Caisson No 1 seen in the Bramley-Moore Dock, the other side of the River Mersey in Liverpool.