Here in Michigan, there is a question in regards to whether or not the seller knows of any buried storage tanks. It might not be the case in other states.
I'm assuming that your agent made a few copies of your offer prior to submission, right? One for you, one for them, and one to go to the seller for review. If so, then you can review the disclosure form and see if it was disclosed there.
Sometimes people get upset about such things, but usually AFTER they've already bought the property and are now stuck with having to make amends to it. As John said though, if the seller had no idea that it was there, then they most likely would not be held liable in court unless it could be proved that they knew the tanks were there. And how could one prove what someone else knew or didn't know?