I was exploring a moon last night - had some luck finding scanners and oberservatories, so I had lots of locations pinging in my HUD to go visit.
However, at least two of these sites seemed to be underground (should have gotten a screenshot, sorry).
I parked my ship as close to the marker as possible, jumped out, and there's nothing there... I jumped back in the ship and circled a bit - no structure (even though it was a monolith/ruin marker).
I figured it must be underground - but I couldn't see any cave openings nearby and couldn't be bothered with an extended spelunk. I have seen a couple of sites that are practically inside a cave, but only just on the edge of the cavemouth.
So - has anyone actually visited one of these things underground?