My tank experiences...
Recently there was a post in this forum about buying a house with potential issues like asbestos, lead, and an underground tank. Here is my reply to the tank part.
I switched over from oil to gas heat many years ago, and had the oil tank pumped out and left in the ground. Ten years later, when I decided to remodel and expand the house, the old tank was in the way of the new foundation, so I had to have it removed. When it came out of the ground, it had holes in it and was leaking oil. Fortunately, it was in clay and, after $500 of soil testing, it was certified that the oil had not migrated. I asked the tank removal company about why it still had oil in it, since I had had it pumped out. They said that the oil deliverers never fully pump tanks out because they don't want any of the gunk at the bottom of the tank, and usually leave about 50 gallons in the "empty" tank! If I had any contamination, it would have cost big bucks, and would have delayed my remodel for months, or killed it. Some people have "tank" insurance, which is not always available and is usually only for people with a current oil delivery account.
I have a friend who noticed a little seepage by his oil tank, and, when it was checked out, it was determined that he had an active leak that needed to be fixed. Bottom line: he had to move out of his house for 4 months while the $450,000 cleanup took place. He was lucky: after deliberating for a couple of months, his homeowners insurance company agreed to pay for it. And as for the comment that there is EPA money available to deal with residential oil tanks, that's simply not true. It may be that some state ecology departments may help in some way, but I wouldn't count on it. If you have a leak, you're probably in deep doodoo.
My advice: never buy a house with a "decommissioned" underground oil tank unless the seller can document that it meets local decommissioning standards. I would even suggest making a purchase offer contingent on removal of the tank by the seller and remediation of any contamination at his expense.