Locked in frozen vaults in Antarctica and Greenland, a lost world of ancient creatures awaits another chance at life. Once thought to be too harsh and inhospitable to support any living thing, the polar ice sheets are now known to be a gigantic reservoir of microbial life, trapped longer than modern humans have walked the planet.

With that ice melting at an alarming rate, the earth could soon see masses of bacteria and other microbes the likes of which it has not seen since the Middle Pleistocene, a previous period of major climate change, some 750,000 years ago.

John Priscu, a microbial ecologist at Montana State University, has spent the past 28 austral summers in Antarctica, studying what he calls “the bugs in the ice sheet.” He has found living bacteria in cores of 420,000-year-old ice that are still able to grow and divide.

Do they pose a threat to human health? Not likely, scientists say, because most of what has been identified appears related to common soil and marine bacteria. Still, with heat-trapping greenhouse gases warming the polar regions much faster than the rest of the planet today, investigators have many other questions about these organisms.

Researchers are trying to determine how they can sit in a state of suspended animation for millennia. The findings could point the way for the discovery of life in other extreme climates, such as frozen planets and moons.

But the more immediate concerns sit here on earth. Cells and carbon dumped out of melting glaciers could turn into huge piles of decomposing organic matter that generate carbon dioxide and methane as they decay, a potentially significant source of greenhouse gas emissions that climate researchers have yet to consider. And scientists see evidence that the microbes are evolving inside the ice sheets, exchanging DNA and gaining new traits that could let them take over ecological niches.

Although these cold-loving organisms do not appear to endanger the existence of warm-blooded creatures, they could force out existing microbial populations, with unknown consequences.