Millions of years ago the people of Mars lived and dwelled mainly on the surface, which was lush and fertile, similar but not just the same as Earth.
A little planet that used to orbit between Mars and Earth was affected by a natural event that caused it to deviate from its normal orbit.
It was on a collision course to eventually strike Mars, and the Mars people knew this, so when it was time, some people went underground and the rest left the planet in crafts to wait it out.
The result was much worse than expected because the little planet exploded upon impact, knocking Mars slightly, but so violently, off it's orbit, that it left its atmosphere behind.
Everything on the surface died at once.
There was not a great loss of life of the people, and they continued to live underground.
There they still reside today and have adapted very well.
Living inside of Mars is almost just like living outside, with housing, buildings, roadways, skyways, rivers, lakes and vegetation.