According to the Environmental Protection Agency (EPA), there are approximately 570,000 USTs nationwide that store petroleum or hazardous substances. The greatest potential threat from a leaking UST is the contamination of groundwater, the source of drinking water for nearly half of all Americans. EPA works with its state, territorial, and tribal partners to prevent and clean up releases from UST systems. Most of the petroleum is in the form of gasoline and diesel fuel that is stored beneath retail gasoline stations and prior to 1980, USTs were predominantly constructed of steel. Depending upon soil conditions, some tanks lasted many decades without any corrosion, while at other sites corrosion was present within 10 years. Many thousands of old underground steel tanks have now been replaced with newer tanks made of corrosion resistant materials (such as fiberglass, steel clad with a thick FRP shell, and well-coated steel with galvanic anodes) and others constructed as double walled tanks to form an interstice between two tank walls (a tank within a tank) which allowed for the detection of leaks from the inner or outer tank wall through monitoring of the interstice using vacuum, pressure or a liquid sensor probe. Remediation can be costly depending on the scope of the pollution with the potential for extensive costs for contaminated properties due to large leaks from fuel storage tanks.