What's inside. If we didn't address it and the Iraqis who were supposed to didn't address it I'm guessing it's the sort of horribly expensive mess you get when the barrels decompose. You can't even fill the bunkers with concrete because the stuff will get out. Just a guess.
With all the robots we have I'm not accepting that idea. No solution is prefect, but taking one's best shot usually beats doing nothing. It wouldn't be that hard to make an RC drum-bot or one capable of handling shells. With all the money we were spending, I just don't see leaving it there.
That was my first guess, but why not encapsulate it? Something’s bound to get out when you’re dealing with an end-of-the-world religion.