A period where by earth was covered with ice during a huge ice ages around 650 million years ago. There's also a fair bit of evidence that there was at least one other period around 2 billion years ago.
The hypothesis was only advance in the 1960s, so it's pretty recent. It's thought to be the only explanation for glacially deposited material in the tropics.
Oh, and it got out of it via the greenhouse effect, which went unabated because there was no biological control over the CO2 and methane being pumped out of volcanos. This process might have taken 30 million years! Also the activity of volcanoes darkened the ice and allowed the darker surfaces to absorb more heat from the sun.
Hope that helps! Fascinating stuff!