There is evidence of lava tunnels and some show collapsed parts. The millions of years of thermal cycling with the lunar day/night might have annealed them, making them less likely to collapse and act places where long term sealed/insulated filled with low pressure pure oxygen at about 3 psi(same partial pressure of O2 as earth - less danger of any blow out. I can see the need for permanent roof/wall support as well as a sealed membrane with fiber glass for strength to make them safe for people and sheltered from some impacts