An oldie but a goodie…Area 51:
Easy - Nevada PD
Words: Base, Sand, Cop, Sun, Dry (3-4 letters)
Medium - USAF Base Command
Words: Pilot, Engine, Oxcart, Sonic, Russia (5-6 words)
Hard - Area 51 Memory Banks
Words: Federal, Clandestine, Terrestrial, Unidentified, Investigation (6+ words)