Now that you mention it, Antarctica actually does seem like a good backup solution. Not as good as Mars, and not against Earth shattering asteroids, but much easier to set up.

It's pretty far away from everywhere, so it would be a decent shelter for humans and important resources alike.

Though tbf, good underground bases around the world would work pretty well.

The problem with Mars is it would take hundreds of years with current technology to settle and become self-sufficient. If something wipes out Earth in the meantime, Martians would just die a slower death.

And again, bases on asteroids (especially if they're brought into Earth's orbit) seem like a better idea.