Oil Spill? In my yard?

Apparently when a guy was looking at the house today, with great interest of renting it to me if he purchased it, went on the side of the house no one ever goes on, unless they are mowing the lawn, he saw something very interesting:



The real estate agent was telling me how people in this neighborhood used to keep the oil tanks under ground, they think that the one here burst.. It makes sense but I have no clue, and of course it is just a guess. It is very obviously oil, as you can smell it so clearly the minute you go ever there. I am shocked we never noticed it in the year we have been living here.. I have no clue what to do, it is great because it will probably keep people from buying the house.. But.. it will also probably keep this guy from buying it. And this guy gave me so much hope that we may have a chance to stay in this house. I do not want to move from here.. It is the perfect neighborhood to raise a family. Seeing as Fannie Mae is pretty much my "landlord" as the house is in foreclosure, I have no idea what I need to do to make sure the house is safe for myself and my family.

All I can picture now is the entire house blowing up.. :unsure: