Numerous sea urchins took down a giant kelp forest off the California coast in this video. The purple sea urchins used their sharp teeth to cut down the giant kelp in this timelapse footage from the BBC natural history documentary Planet Earth. Each sea urchin has five teeth which grow continuously and aways remain sharp. Scientists say the roughly 0.8 inch teeth have predetermined weak spots along calcite layers that allow them to 'tear' away but still keep a sharp edge. After the sea urchins attack the fallen kelp forest, known as urchin barrens, cover the sea bed teeming with life.