The Earth’s crust is a visual timeline that goes back billions of years. But all over the world, there’s a gap in the timeline—a huge chunk of crust that should be there yet isn’t. Now, scientists say that the crust may have been destroyed during Snowball Earth, a hypothetical period in which the globe was covered in ice.

The gap in Earth’s timeline is known as the Great Unconformity, and represents 250 million to 1.2 billion years of lost time. The crust timeline abruptly jumps from the Cambrian Period, which saw the most intense burst of evolution, and the Precambrian time, during which the Earth was formed—meaning that it skips over about one-fifth of Earth’s geological history.

So what happened? Well, that crust could have been ripped away by moving glaciers during Snowball Earth, argue scientists in a December 2018 article published in the Proceedings of the National Academy of Sciences. Glaciers’ intense weight causes them to scrape and erode land that they move across. After being scraped up by Snowball Earth glaciers, the missing crust could have fallen into the ocean as sediment, and then disappeared into one of Earth’s lower layers.

The scientists argue that a big geochemical shift happened around the time Snowball Earth’s worldwide glaciers formed, something that suggests Earth’s crust was being recycled. “Although this erosion didn’t apply evenly across the world, it amounts to an average sediment layer 1.9 to 3.1 miles deep being swept away,” reports National Geographic.

If correct, this theory explaining the Great Unconformity could help shed light on other aspects of Earth’s history, like why Earth lost most of its craters around 600 to 700 million years ago.

It could also help connect the dots between Earth’s formation and “Cambrian explosion” of life between 541 million and 530 million years ago. So far, we know there were large animals before the explosion—the oldest is the 558 million-year-old Dickinsonia, a sci-fi looking creature that could grow up to four and a half feet long. But there are still a lot of questions about what was going on before that, in the Earth’s gap years.