Area 51: A Technical History of Secret Aircraft Programs and Uncertainty

To the point

Area 51 in Nevada began in 1955 to test the U-2 for the CIA under Richard M. Bissell Jr., later expanded to support the SR‑71, A‑12, and F‑117 programs, and its ongoing secrecy and lack of official information have helped fuel UFO lore.

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Area 51 at Groom Lake: UFOs or top-secret aircraft

Technical analysis of Area 51 at Groom Lake: classified aviation programs or simply unexplained phenomena?