Backyard findings

So today Rob and I were outside with Seth and Teagen. I was watching Seth put rocks on the deck and then his mouth (ewwwww!) when Rob called me to the back of the yard. I had asked him to plant a hydrangea he had gotten me for my birthday. I had picked out the spot and he was digging when he found an odd hole in the ground. He had only dug down about 6-8 inches and then uncovered a bottomless hole. What are the odds that we should pick the exact spot of this hole. It didn't look like it went side to side. We called our backdoor neighbor over to have a look too to get his opinion.
Doesn't look too deep does it? The shovel is about 5 ft tall, Rob proceded to go down all the way to his armpit and still did not touch the bottom of the hole!!! Lee, our neighbor, suggested we put our hose down it and see if we could touch bottom. We ran the hose down it til we thought it hit the bottom and pulled it out and measured 15 ft!!! Where the heck did it come from? Even scarrier is the fact that the kids play over there all the time and no one has fallen through! Lee grabbed his flashlight and they looked as far down as they could. Deep enough to have water in the bottom. So, we will be calling the county on Monday to see what we should do about it, if anything.