The only thing that stood between Shackleton's crew and the sub-freezing waters of the Southern Ocean was a six-foot-thick skin of sea ice, which is not known for its stability. The Endurance itself was crushed by the force of millions of tons of shifting ice, and Shackleton regularly had to relocate his ice camps as cracks appeared. "[A]s I was passing the men's tent, the floe lifted on the crest of a swell and cracked right under my feet," Shackleton wrote shortly before the team took to the lifeboats. He watched as the crack widened right under the tent, dumping Walter How and Ernest Holness, who was still in his sleeping bag, into the soup. How got out himself, while Shackleton grabbed Holness, bag and all, and heaved him back onto the floe.