It always seemed to me that if we could populate Mars, we should be able to populate a self-sustained under-water or subterranean colony.

Such a colony would provide practice at designing biospheres, and could be pretty robust against space hazards, with much shorter round trip times while testing it, if we're really convinced we need a lifeboat for Earth using existing tech.

(Though I suspect we partly want to colonize Mars because space is cool, which is fine too.)