Doctors Chris and Xand van Tulleken return to Store Glacier in Greenland with a team of top experts and explorers to find out more about glaciers and icebergs. In this episode they find out exactly what makes a glacier slide towards the sea. Diver Andy Torbet dives to the bottom of a blue lake in search of a giant plug hole that could drain the lake at any time and they see amazing time-lapse pictures of the lake which reveal the existence of speedbergs.