Sea ice melting can start a feedback cycle where more ice melting reveals more dark ocean water that absorbs more sunlight and heat, melting more ice. How calm or choppy the ocean is affects how ice crystals form and combine into different types of ice, like thin nilas sheets or ridged pancake ice. Sea ice can be "fast ice" attached to land or sea floor or "pack ice" that drifts with winds and currents, colliding to form thicker ice. Large, persistent openings in the ice called polynyas are maintained by upwelling currents or offshore winds preventing ice formation.