My daughters were playing outside when the ground gave way and one of them, my eldest, almost fell in. Water fills the bottom of this hole, and all together it is only about a couple of feet deep. I can say that my daughter escaped unscathed, only muddying her shoes.
Of course, when you have little ones and something like this happens, questions invariably follow.
"Dad, what if it was filled with lava? Would you make us play in the front yard?" or "Dad, what if it was filled with Kool-aid and it was clean, would you let us drink the whole thing?"
See, even at that age their imaginations run wild, which is a good thing.
Unfortunately, for an adult, in the backyard (and in the case of my writing, a story) the last thing you want to see is a gaping hole.