Operation Wigwam Underwater Atomic Explosion. The photograph shows the explosion plume from a 20-kiloton atomic device detonated one-half mile beneath the ocean surface 700 miles southwest of San Diego in May 1955. The device was suspended from a raft fabricated from empty oil drums, and which was towed upwind behind a navy fleet tug by a seven mile cable buoyed by floats at 200-ft intervals. More than 150 instruments for recording various phenomena were deployed along the cable, including two balloons at 1500 feet above the surface and two scale model submarines 300 feet beneath the surface. SIO participation in this exercise, under the direction of Gifford Ewing, Associate Research Professor, included the M/V HORIZON, which tracked surface radiation and temperature, and to drop the original Van Dorn water samplers that were to be picked up by helicopters for early-time energy analysis. Both aircraft and helicopters operated from the light carrier WRIGHT (CVL49). The plume height is about 1600 feet and diameter one mile. I was aboard the lead aircraft flying directly over the plume at an altitude of four thousand feet when this photo was taken.