TWA Flight 800
On the evening of July 17, 1996, a Boeing 747-100 departed from New York's JFK as TWA Flight 800, bound for Rome, Italy. A little over ten minutes after takeoff, a catastrophic aircraft fuel tank explosion broke the plane apart off the coast of Long Island, killing all 230 people on board. The disaster sparked an exhaustive investigation that lasted over four years, and involved both the NTSB and the FBI. The investigation concluded in 2000, and the NTSB's final report cited the cause of the incident to be an explosion the Center Wing Fuel Tank of the 747.
The fuel tank in question was empty at the time of the explosion, and the air inside was saturated with fuel vapors at very high temperatures. Prior to takeoff, the aircraft had sat at the gate for several hours in the sun with the on board air conditioning systems running. The air conditioning systems can generate a lot of heat and are located directly beneath the center wing fuel tank. The NTSB report states that faulty wiring in the fuel tank was the most likely source of a spark that caused the fuel vapors in the tank to ignite. The explosion, in turn, caused the internal structure of the aircraft to fail, resulting in the in-flight breakup of the airplane.
TWA 800 is not a unique case. According to the FAA, since 1960, 18 airplanes have suffered fuel tank explosions caused by ignited fuel vapors. Many of these explosions were catastrophic in nature.