Put some top soil over it and plant some grass :D
Do anything else with it and the EPA will come out, you will have to pay for a EPA approved Co to dig it up and take care of it, and so on and so on.
Where I used to work at, we leased some land to a garage. After it closed shop, the EPA came out 12 years after to check on the under ground tanks and tell us we had to remove them. They found where an old oil tank had leaked and we had to pay to do the clean up. They took out 18 55gal drums and the clean up guys told us that we would always own the drums and that they just stored them for us :nuts: Today they are in a salt mine under Lake Erie :hmm: