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
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