Ah, the underground Mars colony. It was confusing to me as to why this was so relevant to SpaceX that they'd not only take away engineering resources from other projects, but also start ripping up their parking lot to experiment.

If you want to build a radiation-shielded settlement on Mars, the best bet is to dig. But how do you dig on Mars? Just as Tesla & SpaceX solve the question of electric power in space, this project starts to solves the underground colony question...