Boeing's liquid-hydrogen powered Phantom Eye completed its first test flight in June 2012 at Edwards Air Force Base in California. With its 150-foot wingspan, the UAV climbed to just over 4,000 feet at a speed of 62 knots. Phantom Eye's environmentally friendly propulsion system (its "exhaust" is water) will let it stay aloft 10 miles high for up to four days. But watch out below: Upon landing, the vehicle's landing gear dug into the lake bed and was damaged.
Image credit: Boeing