A rare, translucent, black-and-white crystal that sat in a box for 30 years has led scientists to a startling discovery: The East Antarctic ice sheet, which holds 80 percent of the world’s ice, may be even more vulnerable to warming than once believed.

Scientists had determined that this ice sheet last retreated about three million years ago. But a new paper in the journal Nature suggests—based on a study of crystals collected from the region—that a large part of it collapsed only 400,000 years ago. Most startling of all, the team’s calculations suggest that the dramatic change happened during an extended but relatively mild warm spell.

During that time period, the amount of carbon dioxide in the atmosphere never rose very