A subsea foundation comprises a caisson (16) that is at least predominantly of steel and is preferably made mainly of flat steel plates, using conventional shipyard fabrication techniques. The caisson is lightweight, hence being transportable aboard an installation vessel rather than requiring towing, and can be lifted onto and off the vessel by a marine crane.
The caisson has a base and a peripheral wall (42) upstanding from the base (40) to define at least one open-topped compartment (52) in which ballast (50) can be deposited onto the base after the caisson is lowered to the seabed.
A column (20) like a monopile for a wind turbine is co-operable with or fixed to the caisson (16). The column may be lowered with the caisson or lowered after the caisson and united with the caisson on the seabed. For the latter purpose, the caisson may comprise a tubular socket that receives the column telescopically.