On March 28, 1979, something went wrong inside a power plant on Three Mile Island in Pennsylvania. The plant used nuclear energy -- power created by splitting tiny atoms -- to make electricity. That morning, a small problem caused a chain of bigger problems. Cooling water stopped flowing the way it should. The reactor, the part of the plant where atoms are split, started to overheat. Workers tried to fix the problem, but confusing controls and missing information made things worse. Part of the reactor's core melted. People living nearby were afraid. The governor told families near the plant to leave as a precaution. Pregnant women and young children within five miles left first. About 140,000 people evacuated -- they packed up and left the area. In the end, no one was injured. Scientists later found that only a tiny amount of radiation. Invisible energy that can be harmful in large amounts -- escaped into the air. But the fear was huge. Americans, who had once been excited about nuclear power, suddenly felt unsafe. After Three Mile Island. No new nuclear power plants were approved in the United States for more than thirty years.