Comment by ben_w

The whole thing is sufficiently bizarre that I just have to assume it's an excuse to develop something more useful.

If that something is cheap autonomous mining that can be sent to Mars to build a colony before anyone arrives, or a sneaky way to make very large underground nuclear bunkers that always have a surprisingly large number of random ordinary people in them, or just that Elon knows about a major valuable mineral deposit that nobody else is aware if yet, great. But if this really is just some self-driving pods that attach to your car and take you around at relatively high speeds, I don't see the "while underground" doing much for the congestion.