Fishy in the least! I thought the same thing. I have a cistern that's 20 feet across with poured concrete as the top. Sure, it gives me the willies to walk on it but it has held since 1933. Something there isn't right.
We rented a quarter of ground that was next to ours. It had a mineral well on it. We went to check cows one day and happened to look over and saw a huge sinkhole. At the time it was probably 75 foot across. Maybe 50 feet deep. It was right at where that well was - you could still see the old rusty drill pipe sticking up in the middle. There was NO SIGN of all of that dirt. It was all water at the bottom. We watched it over the next month or so. It probably ended up 200 foot across. Not sure what ever happened as that quarter got sold later. If I mapped it I'm sure it still looks like a pond from space.
Scary thing was that two weeks before we cut wheat on that ground. I sat right beside that well in the grain truck. That made a teenager think, I'll tell you.