Japan has begun the process of pumping over a million metric tons of treated radioactive water from the Fukushima Daiichi nuclear power plant. The water, which was contaminated from contact with fuel rods, will be gradually released into the sea over the course of several decades. Tokyo Electric Power (Tepco), the utility responsible for the plant, has been filtering the water to remove isotopes, leaving only tritium, a relatively harmless radioactive isotope of hydrogen.