Before fifty million years ago, Earth had no regular ice ages, but when we did have them they tended to be colossal. A massive freezing occurred about 2.2 billion years ago, followed by a billion years or so of warmth. Then there was another ice age even larger than the first— so large that some scientists are now referring to the age in which it occurred as the Cryogenian, or super ice age. The condition is more popularly known as Snowball Earth.
"Snowball," however, barely captures the murderousness of conditions. The theory is that because of a fall in solar radiation of about 6 percent and a dropoff in the production (or retention) of greenhouse gases, Earth essentially lost its ability to hold on to its heat. It became a kind of all-over Antarctica. Temperatures plunged by as much as 80 degrees Fahrenheit. The entire surface of the planet may have frozen solid, with ocean ice up to half a mile thick at higher latitudes and tens of yards thick even in the tropics.
There is a serious problem in all this in that the geological evidence indicates ice everywhere, including around the equator, while the biological evidence suggests just as firmly that there must have been open water somewhere. For one thing, cyanobacteria survived the experience, and they photosynthesize. For that they needed sunlight, but as you will know if you have ever tried to peer through it, ice quickly becomes opaque and after only a few yards would pass on no light at all. Two possibilities have been suggested. One is that a little ocean water did remain exposed (perhaps because of some kind of localized warming at a hot spot); the other is that maybe the ice formed in such a way that it remained translucent— a condition that does sometimes happen in nature.
If Earth did freeze over, then there is the very difficult question of how it ever got warm again. An icy planet should reflect so much heat that it would stay frozen forever. It appears that rescue may have come from our molten interior. Once again, we may be indebted to tectonics for allowing us to be here. The idea is that we were saved by volcanoes, which pushed through the buried surface, pumping out lots of heat and gases that melted the snows and re-formed the atmosphere. Interestingly, the end of this hyper-frigid episode is marked by the Cambrian outburst—the springtime event of life's history. In fact, it may not have been as tranquil as all that. As Earth warmed, it probably had the wildest weather it has ever experienced, with hurricanes powerful enough to raise waves to the heights of skyscrapers and rainfalls of indescribable intensity.