The Technical Standards Safety Authority (TSSA) defines underground fuel oil tanks as those that are buried and in direct contact with earth or backfill. They contain fuel oil to be used in appliances, such as furnaces and boilers. The TSSA does not consider fuel oil tanks in basements to be underground tanks unless they are in direct contact with backfill material. Free-standing fuel oil tanks in basements that not in direct contact with backfill material are defined, by the TSSA, as “above ground” tanks. Click Here For More from TSSA (pdf).