[HangnJudge]

Look at the vertical striations on the inner wall...

[Resettozero]

Look at the vertical striations on the inner wall...

Reminds me, for some reason, of Katie Currack.

[Texas Songwriter]

It looks like it has been bored out. I did a lot of post-holes and it has a similar appearance. Look at the “dirt” around to edges of the hole.

[MaxMax]

That's water running down inside of it, add the natural chemicals to the moisture
and it's probably eating the side or melting it.

You can see the top of the hole where the water is running into it, separating,
then making smaller streams.

Or, aliens have invaded and they're going to appear in our toilets all at once.
We'll need "Giorgio Tsoukalos's" opinion on this before we can really know for sure.