‘| 48 RADIOLOGICAL CLEANUP OF ENEWETAK ATOLL Twenty-seven structures of various designs were erected, and blast force and other measurements were made on them.80 Two of the struc. tures constructed for this purpose are shown in Figures 1-46 ang 1-47, OPERATION IVY: OCTOBER-NOVEMBER 1952 There were only two detonations in Operation Ivy, but the first of these, Event Mike, was especially significant as it was the first test of an experimental thermonuclear device. The test occurred on 31 October 1952, and the device (it was not a bomb in the true sense) was located on the surface of Elugelab, one of the most northern islandsof the atoll. The yield was 10.4 megatons (MT), equivalent to 10.4 million tons of high explosives. The general appearance of the device is shown in Figure 1-48, Theisland of Elugelab was practically vaporized by the detonation andin its place was a crater more than a mile in diameter and 200 feet deep. A large fireball, 3-1/2 miles in diameter and followed by a wave of water, swept across neighboring islands. Trees and shrubs facing thetestsite on the island of Biken were scorched and wilted, although they were located 14 miles southwest of the Mike shotsite.8! Figure 1-49 showsthe island chain before the shot. The visible causeways were constructed to carry FIGURE 1-46. HANGARS CONSTRUCTED TO STUDY BLAST EFFECTS, ENJEBI.