The explosion was 960 feet (300 meters) beneath the surface at the Nevada Test site, about 80 miles (130 kilometers) northwest of Las Vegas. "It doesn't shake the ground," Morgan said.

A mushroom cloud is seen after the explosion of a French atomic bomb above the atoll of Mururoa, also known as Aopuni, in 1971. French Polynesia, Pacific. Galerie Bilderwelt/Getty Images ...