An unsettling picture of a US physicist cheerfully holding the 'Fat Man' atomic bomb's core

This image from 1945 shows Manhattan Project physicist Harold Agnew smiling and holding the plutonium core of one of the world's most devastating weapons.

Weighing 14 pounds and responsible for approximately 80,000 deaths, the heart of the "Fat Man" atomic bomb was detonated on August 9, 1945, over the Japanese city of Nagasaki.

Nicknamed after Winston Churchill, "Fat Man" was the second nuclear weapon to be deployed in combat after the US dropped a 5-ton atomic bomb on the Japanese city of Hiroshima.

Agnew, who became the third director of the Los Alamos Laboratory, flew in a plane along slide Enola Gay and took the only footage of the attack from a bird's eye view.

Here is a clip of the film he captured:

Three days later, the US dropped another bomb on Nagasaki which prompted Japan to surrender to the Allied Forces, effectively ending World War II.

Of the 10,800 pound "Fat Man" bomb, approximately two pounds underwent a fission reaction and a gram of that yielded an explosion equivalent to 21,000 tons of TNT, according to the Atomic Heritage Foundation.