The zone that deep-sea fish do not inhabit is the epipelagic zone (0m-200m), which is the area where light penetrates the water and photosynthesis occurs. This is also known as the euphotic, or more simply as the photic zone. Because the photic zone typically extends only a few hundred meters below the water, about 90% of the ocean volume is in darkness. The deep-sea is also an extremely hostile environment, with temperatures that rarely exceed 3 °C and fall as low as -1.8 °C" (with the exception of hydrothermal vent ecosystems that can exceed 350 °C), low oxygen levels, and pressures between 20 and 1,000 atmospheres (between 2 and 100 megapascals).