Former changes in sea levels have left continental shelves, shallow areas in the sea close to land. These nutrient-rich waters teem with life, which provide humans with substantial supplies of food—mainly fish, but also shellfish, mammals, and seaweed—which are both harvested in the wild and farmed. The most diverse areas surround great tropical coral reefs. Whaling in the deep sea was once common but whales' dwindling numbers prompted international conservation efforts and finally a moratorium on most commercial hunting. Oceanography has established that not all life is restricted to the sunlit surface waters: even under enormous depths and pressures, nutrients streaming from hydrothermal vents support their own unique ecosystem. Life may have started there and aquatic microbial mats are generally credited with the oxygenation of Earth's atmosphere; both plants and animals first evolved in the sea.