Well, our first swing set had a well under it!!! You read that right...a well! Things were going along quite nicely for about 2 years. The swing set was in a nice enclosed area about 5 ft. from the house. Then, one spring, our son started pulling at a stick that was coming out of the ground under the swing set. When dh went to get it out, he pulled and pulled and out came a huge branch!! It had been stuffed down a beautiful dry well. Seems we had this dry well for some reason or another and never knew of it's existence until that spring. We then filled in that well with all sorts of stuff. I think dh even put a Big-Wheel down there. I pity the poor person who digs it all up in the future.
My dd's swing set had grass under her swing set...no wells this time, but it is a different house and much newer to boot!!!