Orbiting from above, space probes have located openings to a vaster underground on both the moon, and above, Mars.
This scene (By NASA, JPL, U. Arizona – https://science.nasa.gov/download/file/fid/89311, Public Domain, https://commons.wikimedia.org/w/index.php?curid=87679776) is to be found on the slopes of one of the Red Planet’s four massive volcanoes, Pavonis Mons. I wouldn’t want to be a rover controller tasked with roving up to the edge for a look-see. Better to send in a copter.
Just for scale, that opening is about as large as a baseball diamond, and leads to an underground lava tube, now empty of melted rock. Depending on the state of affairs at the ring, it might be an optimal place for future explorers to dome over the opening, pump some air inside and have a nice, large habitat safe from most of the sleeting radiation from the sun.