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?