astroengine writes: "Recent observations of the lunar and martian surface are turning up multiple discoveries of "skylights" — collapsed roofs of hollow rilles or lava tubes. These holes into ready-made underground bunkers could provide ideal shelter for future manned bases on the two worlds. Firstly, they would provide shelter from the barrage of micrometeorites, solar x-rays and deep space cosmic rays. Secondly, they'd help protect our burgeoning colonists from the extreme swings in surface temperature (on the moon, temperatures vary by 500 degrees F, but inside these lava tubes, the environment remains at a fairly constant -35 degrees). Thirdly, the sci-fi notion of underground space cities could become a reality."