Imagine a world without liquid water, just solid ice in all directions. It would certainly not be a place that most life forms would like to live. And yet our planet has gone through several frozen periods, in which a runaway climate effect led to global, or near global, ice cover. The last of these so-called "Snowball Earth" glaciations ended around 635 million years ago when complex life was just starting to develop. Early models showed that once ice reached tropical latitudes, a positive feedback loop would take hold, in which ice cover would lead to lower temperatures, which would add more ice cover, which would lower temperatures even more. This runaway effect would presumably continue until the entire planet froze over, with even the oceans covered with as much as a kilometer thick layer of ice. This so-called hard “Snowball Earth" would lock the planet into an eternal winter with no apparent way to escape from such a deep freeze. Indeed, scientists have had a hard time explaining how a hard snowball could ever thaw. One proposal is that volcanic activity releases greenhouse gases that eventually warm the planet back up. The amount of carbon dioxide (CO2) needed might be several hundred times higher than what our atmosphere contains now. The other method might be a giant asteroid strike, as is being proposed by Curtin University scientists in Australia.