By now you probably know that the tank has to be removed and the ground around it inspected and tested for contamination, which isn't cheap. Like any other problem found during inspection, you should have the opportunity to cancel the deal if you're still in the inspection contingency period or amend the contract so that removal and a clean inspection report are requirements for closing.
Best of luck!
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?