At around 7am on a Thursday morning in June 1991, people all over Southern California were awakened by a loud boom. It was enough of a shock to make people call their local radio stations to ask whether the noise had been caused by an earthquake; but it had not.
The boom was unusual in that military pilots know booming urban areas is bad for public relations. It was also unusual in that it could be tracked and measured unlike any other way anywhere else in the world. Waiting for signs of the next massive release of tension in the San Andreas fault is a full-time job for United States Geological Service (USGS) seismologists based at the California Institute of Technology in Pasadena. The CalTech seismologists monitor and record every slight movement in the ground with an array of over 220 remote seismographs from the southern coastline to the eastern Mojave Desert.
During the 1980s, the USGS found that their instruments would respond when NASA space shuttles descended overhead on their supersonic approach to Edwards AFB. They also realized that they could track the shuttle's track and speed by comparing the time at which the boom arrived at different points.
The USGS calculated that the June 1991 boom, and other booms tracked between October 1991 and June 1992, were produced by something moving between Mach 3 and Mach 4. The space shuttle flies much faster during re-entry, but no shuttles were landing on those dates. The track calculated ran northeast over Los Angeles, across the Mojave Desert and towards somewhere between Death Valley and Las Vegas. Indications of its destination was that of the Nellis Range, the largest area of military testing and practice in the world.
The unidentified aircraft that boomed L.A. in 1991 and 1992 were not heard from again beyond Nevada. They were most likely headed for the top-secret Groom Lake installation. If they were headed for Groom Lake, some 300 miles from Los Angeles, even at their record-breaking speed they were already decelerating.
The Los Angeles booms weren't an isolated incident. They were the first corroborated evidence to support rumors, sightings and hearings that had accumulated since the mid-1980s. The first reports were heard in 1986, not long after a large section of land was sealed off to observers on public land. Clearly, it was felt that the base's security was inadequate.
In February 1988, the New York Times reported that the USAF was working on a stealth reconnaissance aircraft capable of Mach 6. Early in 1990, Aviation Week also made reference to such an aircraft, reporting that witnesses had heard an incredibly loud aircraft departing from Edwards AFB at night.
Considering the coincidental timing of the retirement of the SR-71 Blackbird in 1990, the newspaper and magazine reports of aircraft characteristic of hypersonic propulsion, the expansion of the Groom Lake facility and the Los Angeles booms, it isn't unreasonable to assume that a secret hypersonic plane is operating from a secret Nevada location, and now flying operations around the world.