Which planet?
If its Prime its probably the volcano caldera. Yes, there is something down there.

Sounds like a meteor crater.
https://planet-crafter.fandom.com/wiki/Meteor_Crater

Later there will be a lake with a small island (and a survivor's base underwater).

Right, meteor crater not a volcano. Same place wrong name.
The clouds will eventually go away.

Originally posted by Coops:

Right, meteor crater not a volcano. Same place wrong name.
The clouds will eventually go away.

You can go down there before they do though, in some ways its easier to access the goodies on the bottom of the crater before it floods rather than after. Just be sure to take lots of air and water with you, and some food of course too. If you've got the rover equipped with oxygen, even better - keep it in your backpack until you need it.

K thanks all, but when does it flood?

Originally posted by duffysteam:

K thanks all, but when does it flood?

breathable atmosphere is when it clears up i want to say. it's perfectly fine to explore before it clears up but you may want to build a beacon to help avoid getting lost if you haven't been in there before.

Originally posted by duffysteam:

K thanks all, but when does it flood?

Shortly after the rains begin to happen the dust-and-steam clouds will start to slowly go away, and the crater will begin to fill at around the same time. There is a rather short window when it is clear to the bottom and still not flooded - but it does occur, the trick is catching it at the right time.

Not that you can't explore it once it's flooded - you can - but swimming is slower than running and takes a little more oxygen. Not really a lot of difference, actually. :-)

There's an old habitat at the bottom of the crater, and a bigger, slightly newer one on the side, maybe a third up from the bottom or so, plus there's at least three or four blue crates scattered about here and there...