looks like a cave at the bottom on your first pic—
Probably was part of an underground river, and then there could also be a possibility such river dried up many years ago and this may again be possibly an ancient filled in well.
Massive rains? Long ago dormant underground river? I know not a whole lot about the weather down there but they say it was the heaviest in history I think I read somewhere.
Yeah ... it’s too bad we don’t have more pictures at different angles to make it out better ...