Abstract: An underground storage facility for various liquids, such as gasoline, has a cylindrical storage tank within a substantially rectangular vault. The bottom wall of the vault is inclined downwardly, and any liquid leaking from the tank will run down the bottom wall and accumulate in the lower end of the vault. A vertical pipe is supported within the vault, such that the lower end of the pipe is raised just above the inclined bottom wall of the vault, and such that the upper end of the pipe extends through the vault and above the ground. A rod or other implement may then be inserted into the pipe to detect the accumulation of liquid in the lower end of the vault. The top wall of the vault has a removable manhole cover providing access to the interior of the vault for in situ repairs to the tank. The entire assembly is an integral fabricated structure that is lowered as a unit into the ground.