The oceans' shores are constantly in motion. They are a mixture of both land and sea and never the same from one moment to the next. The boundary between land and sea constantly changes with the tide's rhythmic rise and fall. When the tide retreats, seawater trapped in depressions in the rocks forms tide pools. These shallow pools and surrounding areas uncovered by the retreating tides are often teeming animals and plants, which must adapt to environmental extremes to survive.