That's what happened to the "Logan Triangle" in north philly (if you were ever browsing google maps and wondered why there were randomly a bunch of completely empty blocks with a full street grid. Except there the creek that had been there (40 feet deep in places) was not only filled in, but filled in with coal ash which is basically the worst possible landfill material - it's not at all dense, erodes easily, and is both flammable and creates flammable gas