On this day in 1979, a pressure valve at the Unit-2 Three Mile Island nuclear facility failed to close, leading to overheating at the facility and, eventually, an evacuation of surrounding areas in Pennsylvania.

The accident began in the morning, when cooling water drained from a broken pressure valve into nearby buildings. With the water drained, the nuclear core of the power plant began to overheat. Emergency cooling pumps were in place, but human operators shut them off during the initial confusion of the accident. It took until 8 p.m. for the operators to return cooling water to the core and begin lowering the pressure in the reactor.

Although the Three Mile Island power plant was commended as a strong, dependable source of alternate energy, its failure is marked as the worst nuclear accident in the United States. The real crisis in the cooling water incident came two days later on March 30, when operators found a dangerously volatile bubble of hydrogen gas had leaked radiation out of the plant. Pregnant women and young children in the surrounding area were advised to evacuate.

The reactor was never used again, although the nearby Unit-1 reactor at Three Mile Island was not affected by the accident and reopened in 1985. The damaged reactor underwent cleanup until 1990, but was determined to be too damaged be recovered. By this time, the name of the facility had become synonymous with the threat of nuclear disaster.