b'P ROVING THE P RINCIPLEHuge Powder Blast Ro c ked Wide Are afrom Arco Advertiser, August 31, 1945A huge chocolate colored cloud of dust, shaped like a Promptly at 9:30 the signal to touch off the explosion wasmushroom, followed by two muffled thuds, was what a radioed to the control dug-out and the first sight was a huge,l a rge group of navy and army officials, scientists, pho- billowing cloud of dust which resembled an enormous mush-tographers and others experienced Wednesday morning room.In approximately 15 seconds the explosion waswhen a cache of 250,000 pounds of TNwere exploded on heardtwo muffled reports.Capt. Walter Brown, comman-Tthe desert near the gunnery range.For several days all of dant, was certain that the second blast heard was an echo.Eastern Idaho excitedly awaited theblast, as this was the largest collec- The huge cloud of dust seemed totion of powder ever set off by man. hover over the spot for about tenminutes and gradually moved west-On the desert area about 10 miles ward toward the Arco community, aseast of the gunnery range the navy the air currents destroyed the sym-had constructed four igloos.Tw o metry of the cloud and it began towere of a design approved by the resemble a huge dust storm.army and two were of a navy design.They were in proximity of about 500 An inspection of the spot where theyards.Each was built of reinforced Courtesy of Orville and Margaret Larsen igloo stood showed a pit approxi-concrete and stored 250 thousand mately 15 feet deep which waspounds of powder.The one exploded strewn with large chunks of rein-was the army type, and the purpose forced concrete, steel and wire rein-of the experiment was to determine forcing material.Most of the steelthe strength of the igloos and how far surrounding the bombs of TN Tthe earth would be shaken. seemed to have been completelySeismographs had been placed at var- destroyed, although small fragmentsious places in the surrounding area, were found a half mile distant. and a series of blast meters, like U.S. Army Corps of Engineers, Rock Island Districtspokes in a wheel, extended from theigloo area to a distance of approxi-mately one-half mile.Above. Cloud from one of the mass detonationWithin close proximity of the blasted experiments. Middle. Largest of the craters inigloo were installed concrete shelter Mass Detonation Area was later enlarged andproofs for military photographers, and graded by IDO for use as a demolition range.about a quarter of a mile distant was Photo circa 1995. Below. Ruptured and emptythe dug-out where the men who set cans of smokeless powder partially fill a crater.off the charge were housed. The dug- U.S. Army Corps of Engineers, Rock Island District Photo circa 1995.out was of reinforced concrete, and sandbags protected the entrance-way.1 6'