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://.
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Looks like a scene from the movie Alien. Are you in an area with lots of limestone?

Yes anthropic, all limestone.

If you find and plug up the other end of the cave, you might be the only person on Pond Boss with a 200' deep pond!

That's crazy cool, and annoying at the same time!

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.

Limestone will do that - see sink holes in Fla for example. Same thing on your pics.

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.

The opening to this is only 4" . We did take some videos and got down pretty deep, but was not able to follow the bend it took.

ewest, The difference in my part of the world is we only have 2-3" of soil over the limestone.

I did a lot of online searching when this happened, it seems this area has what they call karst limestone.
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Tony,

You creating the forage pond, plus the weight of the water may have contributed to breaking through the crust above the karst feature.

Good thing it was only 4" wide and not a vehicle swallower.

You might need to do a walking survey to determine if you do have any other potential danger spots on your property.

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.