What is pack ice?

Large blocks of floating ice, called pack ice, cover much of the Arctic Ocean and the seas around Antarctica. Pack ice was a problem for explorers. In 1915, the Endurance, the ship used by Sir Ernest Shackleton to reach Antarctica, was crushed by pack ice and sank. The first ship to reach the North Pole was the Russian atomic icebreaker Arktika in August, 1977.