Any ideas on what could this be? It is located behind an 1850's farmhouse, and it is made of old cement. It measures 4 feet wide by 9 feet long by 5-6 feet deep. The walls are 5" thick, which makes the inside about 3'2" by 8'2". It is rounded at each narrow end.

We have filled it in with dirt and turned it into a garden when we moved in, because it was too dangerous to have with little children around.

Sorry, I tried to add pictures, but it keeps telling it's not a valid image file. I wil ltry to include a link to my Facebook photo album:
https://www.facebook.com/media/set/?...1&l=d79ecf5d67