Seventy years ago, on July 26, 1946, the U.S. military tried a new type of nuclear test.

A joint Army/Navy task force had suspended a nuclear device, oddly named Helen of Bikini, 90 feet below the surface of the water, in the middle of Bikini Atoll, one of the isolated rings of coral and land that make up the Marshall Islands. Arrayed around the 21-kiloton bomb were dozens of target ships.

The Navy had a point to prove. In this new era of nuclear warfare, in which the Air Force could rain down explosives on entire cities, what use was a naval force? The military leaders who proposed the test wanted to show that their ships could ride out a nuclear attack and that the fleet was not obsolete.

But the underwater test was controversial, perhaps even more so than land-based test blasts. Even nuclear scientists questioned its point—would it offer useful, scientific information or was this all just for show?

When Helen of Bikini exploded, it created a giant, underwater bubble of hot gas. In seconds, the bubble hit the seafloor, where it blasted a crater 30 feet deep and at least 1,800 feet wide. At the same time, the surface of lagoon erupted into a giant column of water, 2 million tons of it, which shot more than 5,000 feet into the air, over an area a half-mile wide. In the seconds after the blast hit the surface, a cloud of radioactive condensation unfurled across the lagoon, hiding the column of water shooting upwards; at the top, a mushroom cloud of gas bloomed against the sky.

Read More- Atlas Obscura

Image courtesy of US Army