It may sound unlikely but after seeing the evidence and the interviews with experts it seems very plausible.
The US goverment denys they exsist, but they denied the existence of the blackbird for 3 years!
Heres what its meant to look like:

Heres a few quotes from various places:
"On several occasions from June 1991 to June 1992, sonic booms were heard over Southern California. They were not produced by any officially acknowledged military flight (which are always careful to remain subsonic over urban areas). The booms were powerful enough to show up on the seismographs operated by the US Geological Service, and the times of arrival of the sound at various points allowed fairly accurate calculation of the course and speed of the aircraft responsible; the USGS had already demonstrated this by tracking incoming space shuttles. The aircraft were headed northeast, over Los Angeles and the Mojave Desert, towards either the Nellis Air Force Base in Nevada or the nearby Groom Lake base. The speeds involved ranged from Mach 3 to Mach 4. "
"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."
"In February 1988, the New York Times reported that the USAF was working on a stealthy reconnaissance aircraft capable of Mach 6. The story was attributed to "Pentagon sources". "
"In 1985, a censor's error let an item labelled "Aurora", with no further explanation, appear in that year’s Pentagon budget request, with a reference to "production funding" for 1987. It was located next to the operating budgets for the SR-71 and U-2. The Pentagon refused to comment on the item, and it has never been mentioned since."
Few links of interest
http://wave.prohosting.com/aurora85/art ... urnal.html
http://wave.prohosting.com/aurora85/art ... uakes.html
http://www.google.co.uk/search?num=100& ... arch&meta=