Although less well known than
Olympus Mons, the largest known volcano in the Solar System (at least by

some measures), there are
other enormous shield formations on the Red Planet, including the 18 kilometre high
Ascraeus Mons—named in 1973 for the rustic birthplace of Greek poet Hesiod, which has yielded recently some rather amazing imagery of its terrain to Mars Express of “sinuous rilles,” features thought to be collapsed lava tubes. Not only towering by terrestrial standards, the gently sloping flanks cover a huge area, a footprint roughly the size of Romania or the state of Arizona. More from Universe Today at the link above.