My friend is doing a campaign of Pendragon set on a distant future mars with Mechs. I am doing a lot of thinking about caves.
As if by sheer chance I stumble across the latest issue of 'Sky and Telescope', which has within, an article by Astronomer and caver Robert Zimmerman.
Italics are mine.
"Scientists have so far identified eight different features resembling lava tubes in the Arisa Mons are alone, ranging in length from 6 to 60 miles (10 to 100km), each having from one to dozens of skylight openings. These entrances are generally less than 20 feet (6 meters) across with depths ranging from 30 to 100 feet.
The second type of cave in this area is also volcanic, but is more complex than a lava tube. Here, the tubes are associated with fractures or cracks. These caves probably formed when the Tharsis Bulge began to rise, causing the crust to crack. As lava flowed up into these cracks, the tops crusted over. When the molten lava drained away in various places, it left behind caves, just like lava tubes. Unlike lava tubes, however, these caves are not sinuous; they follow the fracture lines. These caves may extend downwards as much as three miles.
Members of the third type, dubbed "atypical pit craters," are generally larger and deeper than the common lava tube pit entrance, sometimes with diameters as large as 1000 feet. They also have very steep vertical walls, sometimes wth signifigant overhangs that suggest lateral passages of unknown extent. These atypical pits are circular and resemble impact craters at first glance, but they were not formed by impact. Though geologists have not yet determined the actual formation process."