Operation Crossroads was a series of nuclear weapon tests conducted by the United States at Bikini Atoll in mid-1946. It was the first test of a nuclear weapon after the Trinity nuclear test in July 1945, and the first detonation of any nuclear device following the Fat Man detonation on August 9, 1945. Its purpose was to investigate the effect of nuclear weapons on naval ships. 
Fifty-seven guinea pigs, 109 mice, 146 pigs, 176 goats, and 3,030 white rats had been placed on 22 target ships in stations normally occupied by people.[48] Ten percent of the animals were killed by the air blast, 15% were killed by fireball radiation, and 10% were killed during later study. Altogether, 35% of the animals died as a direct result of blast or radiation exposure.[49] The most famous survivor was Pig 311, who was found swimming in the lagoon after the blast and was brought back to the National Zoo in Washington, D.C. 
By August 3, Colonel Warren concluded the entire clean up effort was futile and dangerous.[107] The unprotected sailors were stirring up radioactive material and contaminating their skin, clothing, and, presumably, their lungs. When they returned to their support ship living quarters, they contaminated the shower stalls, laundry facilities, and everything they touched. Warren demanded an immediate halt to the entire cleanup operation. He was especially concerned about plutonium, which by weight made up 78% of the contaminating material and was undetectable on site.
source wikipedia

As horrific as the Atomic bomb is these tests did provide some amazing and terrifying pictures. Operation Crossroads was also later referred to as the worlds first nuclear disaster (tell the people of Hiroshima that) due to the botched clean up effort and exposure of a huge number of military personnel to radiation...

Previous post on French tests at Licorne in French Polynesia here