I thought I would share this, I found it very interesting. This happened about two years ago, after work I topped the hill to our little forage pond and it was almost completely drained!(.10 acre) I found a hole I could stick my arm into, I ran a tape measure as far as I could, it read 21'! .I had to know what was down there so I ordered one of those snake cameras and got my son to hook it up. Here are just a few of those pictures.http://. http:// http://
The temptation would be to get an even longer endoscope and see if it connects to something even larger. The interesting part is the walls of that thing look like they have seen a lot of water flow and deposits over the years. It may have been part of a spring or small cave system once, where now water flows down at a deeper level or was cut off by construction.
If it is downward enough you could get an old GoPro ad drop it down on some masonry string as far as it will go while recording.
Is it possible this is the remains of a dinosaur. Looks like the cavity of a large animal. Real just kidding. I would b tempted to try and keep tht feature visible if possible. You could ad a light to the inside. How wide is it. Can you take some pictures alil farther back from the opening. It's hard to tell the size of this.
Yes, we were all over that area with heavy equipment(yikes) digging base material for our driveway, thats how the forage pond got its beginning. It rained and it held some water all summer, so we worked it into a little pond.