The picture below shows an old severely corroded fuel oil tank in the basement of a residential home. In order to fully inspect fuel oil tanks an inspector must look at the bottom of the tank. This can be difficult in some cases. Many times an extendible large mirror is needed.

This photo was taken from the basement floor looking up underneath the tank. The black "gunk" hanging down from the tank is actually solidified fuel oil, that has seeped through the tank. The bottom of this tank was severely corroded (soft). The homeowner of this property risked the danger of this "gunk" getting pushed out of the tank and allowing the basement to flood with fuel oil.