The ice pack is multiform and evolving. In the Arctic, there is the shore-fast ice which is seasonal and melts in summer, and the pack ice which is in constant movement in icy waters. Fast ice consists of polynyas, areas of open water due to the circulation of warm currents, and open water channels formed by the breaking of large ice sheets under the influence of currents and winds movements. Sometimes covered with solid snow, these are formidable traps for man.