Comment: These holes take a very very long time to form.

(See in situ)

In reply to comment: Here's some data on Florida. (see in situ)

These holes take a very very long time to form.

The acid in our surface water is tannic acid. It eats away at the limestone beneath the ground,s surface. The acid comes from oak and other tree leaves. Especially oak. The sheet flow has a red hue from the tannins. It always has - or so long as humans have been here. The sink holes take roughly a million years to form. The collapsing is what we witness. There are two kinds. A rapidly collapsing variety and a slow collapser. They both are formed the same way and over the same amount of time, but collapse at different rates.