The Aurora 1990 GROOM LAKE, NEVADA, USA In the early hours of the morning on January 30, 1992, people all over Southern California were awakened by a loud boom. It was not the first to rattle window panes in Los Angeles, nor has it been the last. US Geological Survey specialists looked at their seismographs, which registered the event as if it had been another earth tremor. A few hours later, they decided the disturbance had been caused by an unidentified flying object traveling at Mach 3.1. They even figured out where it was heading - straight for the US Air Force's top secret Groom Lake base in Nevada. In spite of official denials, the evidence indicates that the United States is operating a very fast, very secret spy plane. Most people call it Aurora, after an item listed by mistake in a 1985 Pentagon budget document, though it most probably has another code name. The strongest evidence for the existence of Aurora is that the US Air Force retired its Lockheed SR-71 Blackbirds in 1990 without replacing them. The Air Force says its spying can be done by U-2s and satellites, but neither are as responsive as a supersonic highflying craft such as the SR-71. Eye witnesses have seen unusual triangle-shaped aircraft in flight over the United States and the United Kingdom's North Sea. Several have heard a distinctive low frequency rumble followed by a very loud roar, which could be the exotic engine used by a Mach 6 (4,400 miles per hour) aircraft. Experts say a methane-burning combined cycle ramjet engine (uniting rocket and ramjet designs) could have been developed to power Aurora. Observers in California have also reported seeing a large aircraft with a delta-wing shape and foreplanes. Some think this could be an airborne launch platform for satellite-delivery rockets or even the Aurora, before its more advanced engines were developed. No matter what speculation takes place, it seems the secrets that lie beyond the mountains of the Nevada desert will remain until the US military decides otherwise.