Black Hole(s) of Mars

This ought to energize the Hoaxlands and assorted Guthballs out
there: a series of what appears to be bottomless pits, or nearly
so. Deep enough that scattered skylight can't illuminate the
bottoms and extremely steep walls; even on the highest resolution
images they appear as black holes.

http://www.universetoday.com/2007/05/25/dark-caverns-discovered-on-mars/

Hollow Mars, anyone? Now we know where the flying saucers issue
from...

--Damon, fighting holes in his truck's cooling system
.