[ceejayoz]

https://en.wikipedia.org/wiki/Martian_lava_tube have also been proposed as pre-existing possible shelters.

https://en.wikipedia.org/wiki/Caves_of_Mars_Project#/media/F... "HiRISE image of Mars hole 'Jeanne', about 150 meters (492 feet) across and at least 178 meters (584 feet) deep."