If you think Mars is a hostile place now, it was harsher 600,000 years ago.

Scientists say it was stormier and dustier. That’s because there was more carbon dioxide in the atmosphere. Their findings are based on the discovery of a huge underground reservoir of dry ice in the Martian south pole.

Reserachers say that dry ice, or frozen carbon dioxide, used to be in Mars’ atmosphere. Roger Phillips of the Southwest Research Institute says Mars’ climate was probably a lot like the American Dust Bowl of the 1930s — but a lot worse.

There is an upshot. The thicker atmosphere back then means there were more regions on the planet where liquid water probably existed.

The research was published online Thursday in the journal Science.