There's no reason there wouldn't be multitudinous caves on Mars, in fact they should be commonplace, though their aspects may not be what we expect.

But note the description of the formation of this overhang in Noachis:

...a trough, which is part of a large pit crater complex in Noachis Terra. This type of trough forms through the collapse of surface materials into the subsurface, and often begins as a series of individual pit craters. Over time, continued collapse increases the diameter of individual pits until finally, adjacent pits merge to form a trough such as the one captured in this image.

which of course implies subsurface voids, or in other words caves.

Much of the subsurface of Mars appears to be riddled with these voids or caves, and there are plenty of surface pits and hollows showing to some extent what lies below.

Shouldn't it be possible to seal off one of these voids from the surface atmosphere (or lack thereof) and make it a habitable space for earthlings in the great by and bye when we have a space exploration and colonization program again?

Just a thought....;-)