The deep pits on the flanks of Arsia Mons have attracted special interest because some are potentially linked to subterranean caverns. Such locations would be protected from the radiation, impacts, and extreme temperature variations seen at the surface of Mars and therefore could be more favorable for life (including future human explorers).
The three pits seen in this area are in a line, suggesting that they all formed in response to the same underground feature. One possibility is a lava tube, but lava tubes tend to have bends and curves. Furthermore, HiRISE shows that there are multiple layers exposed in the walls of the pits. While it is perfectly possible for skylights into lava tubes to cut through multiple lava flows, this is not the most common case (usually they cut through just the top of a single flow). Hence it is more likely that these pits overly a fault system where the ground is being pulled apart.
In places the ground has collapsed into the resulting void, producing these pits. Such faults are expected as the great weight of these giant volcanoes slowly tears them apart. Similar faults and pits are found on many of Earth's volcanoes, with Kilauea Volcano on Hawaii being perhaps the best studied example.
Photo credit: NASA/JPL/University of Arizona
Note: For another photograph and more information about these same pits on Arsia Mons, see Chain of Pits on Arsia Mons.