Many thank to turketron over at AINC for finding this on wikipedia.
U.S. Swan Device: 1956
The first device whose dimensions suggest employment of all these features (two-point, hollow-pit, fusion-boosted implosion) was the Swan device, tested June 22, 1956, as the Inca shot of Operation Redwing, at Eniwetok. Its yield was 15 kilotons, about the same as Little Boy, the Hiroshima bomb. It weighed 105 lb (47.6 kg) and was cylindrical in shape, 11.6 inches (29.5 cm) in diameter and 22.9 inches (58 cm) long. The above schematic illustrates what were probably its essential features.
Eleven days later, July 3, 1956, the Swan was test-fired again at Eniwetok, as the Mohawk shot of Redwing. This time it served as the primary, or first stage, of a two-stage thermonuclear device, a role it played in a dozen such tests during the 1950s. Swan was the first off-the-shelf, multi-use primary, and the prototype for all that followed.
After the success of Swan, 11 or 12 inches (300 mm) seemed to become the standard diameter of boosted single-stage devices tested during the 1950s. Length was usually twice the diameter, but one such device, which became the W54 warhead, was closer to a sphere, only 15 inches (380 mm) long. It was tested two dozen times in the 1957-62 period before being deployed. No other design had such a long string of test failures. Since the longer devices tended to work correctly on the first try, there must have been some difficulty in flattening the two high explosive lenses enough to achieve the desired length-to-width ratio.
Just to tie things in, remember the Swan station was the original hatch. There was something encased in concrete below it which Sayid said was reminiscent of Chernobyl.
Thanks again turk.