## What the Ice Is
Understanding the Weddell Sea pack ice requires understanding something about how Antarctic sea ice behaves, because the behavior of the ice is what determined everything that happened to the Endurance.
The Weddell Sea is a deep embayment in the Antarctic coastline, bounded on the west by the Antarctic Peninsula and on the east by the Coats Land ice sheet. In winter, its surface freezes into a pack ice field that extends hundreds of miles from the coast. As temperatures warm in the spring and summer, the ice breaks up from the margins, and leads — channels of open water — form through which ships can navigate.
But the Weddell Sea’s pack ice is not simply a solid sheet that melts from the edges. It is composed of individual floes, large and small, that are in constant motion relative to each other, driven by wind and current. These floes press against each other when their paths converge and pull apart when they diverge. When two large floes converge on a ship, the pressure they can generate is enormous — far beyond anything a wooden hull can resist.
The season of 1914–1915 was, by historical comparison, an unusually severe ice year. The pack was heavier than expeditions earlier in the century had encountered. Shackleton pressed into the pack in early December 1914 because the alternatives — waiting for conditions to improve or abandoning the attempt for the season — were unacceptable. The expedition had taken years to organize and fund, the winter base on the Antarctic coast needed to be established before the polar night descended, and he was already running behind schedule.