I Was at a dump about 2 months ago from around 1890 where they filled in part of a swamp. at one end on top of the dump I was looking over the edge & the swamp was about 30 feet below me. Meaning the dump had to be at least 30 feet deep at the far end. at the other side of dump was only 10 feet deep. I only dug 3 feet down, got 1 blob top quart beer bottle & then we left. was very hard dirt & needed a Bulldozer. LEON.
The old town dump in the next town over from me has deep holes like in this video and you can see many places where diggers have tunneled at the bottoms of the 7-8 ft. holes and even partially under tree roots. I just cringe when I see that. Just asking for a cave-in.