What is it?

It is very deep water well in Veszprem - Hungary.

I thought of a pipeline, too, but it looks as if there's a guard rail of some sort across the opening, which is why I went with a well. It almost looks like cobblestones--or, I agree, at least stones--very strange! :-)))

Probably a vertical well with the sun shining on the water at the bottom....or......a horizontal
tunnel.....don't know what it is for in that case. Nice stone work.

Maybe we're not looking down but across...is this a tunnel or pipeline of some kind? You've got my curiosity up! ;-)

It might be a well, but it's very bright at the bottom... Interesting!

OK

Appears to be a well. Fun puzzle!