When a giant (225 square mile) slice of Antarctica's Pine Island Glacier broke off in 2015, scientists wondered exactly what caused it. Well, they now have an explanation... and it's not very reassuring. They've determined through satellite imagery that the break started when a rift was formed at the base of the West Antarctica Ice Sheet, almost 20 miles inland, in 2013. Most likely, warming oceans intruded the sheet at the bedrock well below sea level, triggering cracks that gradually made their way upward. In other words, Antarctic ice could be much more susceptible to breaking up than it seems on the surface, and that separation may be happening faster than researchers expected.