Ice floes, packs, and bergs can also be located and tracked from weather spacecraft. It inhabits ice floes and waters of the north Pacific Ocean and adjacent seas. Drift ice is a floating field of sea ice composed of several ice floes. Pups are born on small drifting ice floes in shallow waters, usually weighing around . Similarly, ships can get stuck between drift ice floes. Ice floes on the Volga now prevented boats and tugs from supplying the Soviet defenders. They would also drop bombs on frozen rivers to break up ice floes. The ships were forced to zigzag to avoid ice floes. By the 1630s whaling had spread offshore and into the ice floes west of Spitsbergen. During the winter, they disperse to the pack ice, ice floes, and the open sea. Drift ice consists of "floes", individual pieces of sea ice or more across. The seascape is shown heavy with ice floes. The reverse is dominated by a pattern resembling the labyrinth formed by adjacent ice floes. "Helix" helicopters are used for scouting expeditions to find safe routes through the ice floes. In 1875 ice floes destroyed the bridge to Matamoras, Pennsylvania.