Abstract: A submersible oil-storage tank is constructed from a foundation plate which has a flat lower face and a concave upper face and contains a series of hollow cells, a cupola mounted on the foundation plate to form a spheroidal chamber, and extensible-retractable tie members connecting the cupola with the foundation plate. An expansible separating wall extends across the chamber and divides it into a lower sub-chamber and an upper sub-chamber. Each sub-chamber is provided with a valved duct so that water may be permitted to flow into or out of the lower sub-chamber and oil may be permitted to flow into or out of the upper sub-chamber.