Coast Guard caused freak explosion. What has your service abondoned that might come back to haunt people?

The Coast Guard dumped batteries from buoys for years. They have spent millions diving and picking up those old batteries after realizing that dumping toxic batteries wasn't very responsible. Abondoned copper cables would never have been thought to be very hazardous though. Actually, they are extremely valuable if you can find one and legally sell it. I had friend who found an abondonded lighthouse cable, dug it up and sold it for scrap. I think he got $10k for it and that was many years ago.

Now that story is something else. The Army has left unexploded ordinance all over he world. I'm confident that those will be haunting folks for a long time to come.

Oh, I don't know...nukes maybe?