At 6:45 a.m. on March 1, 1954, the blue sky stretching over the central Pacific Ocean was split open by an enormous red flash. Within seconds, a mushroom cloud towered 4 miles high over Bikini Atoll in the Marshall Islands. Scientists had underestimated the size of what became known as the Castle Bravo" test, resulting in an explosion that was 2 times larger than expected. Radioactive ash dropped more than 7,000 square miles from the bomb site, caking the nearby inhabited islands. The 1954 explosion was part of nuclear tests conducted [by] the American military. From 1946 to 1958, 67 U.S. nuclear tests pulverized the tranquil reefs and islands of the central Pacific. In 1980, a massive concrete dome - 18 inches thick and shaped like a flying saucer - was placed over the fallout debris, sealing off the material on Runit [Island]. But the $218 million project was only supposed to be temporary. Cracks have reportedly started to appear in the dome. Part of the threat is that the crater was never properly lined, meaning that rising seawater could breach the structural integrity. "The bottom of the dome is just what was left behind by the nuclear weapons explosion," Michael Gerrard, the chair of Columbia University's Earth Institute, told the ABC. "It's permeable soil. There was no effort to line it. And therefore, the seawater is inside the dome." Radioactive material may have already begun to leak from the dome. The Marshallese government, however, does not have the money to shore up the structure.