Thats what I thought. The teacher didn't have a really good answer as to why it wouldn't work. Honestly - this idea nagged at me for years ... until I took geology in high school. Then I got it.
But what do you know ... 20-some years later, and we aren't overrun with trash like "the experts" said we could be.
What was wrong with the idea? Would the waste just become molten and then fill up the hole?