The small, circular hole at the very tip of the tail (the "stinger") of a Boeing 747—and almost all modern jetliners—is the exhaust port for the Auxiliary Power Unit (APU). The APU is essentially a small gas turbine engine, much like a scaled-down version of the massive engines under the wings. Its primary job is to provide power to the aircraft’s systems when the main engines are shut down. This includes providing electricity for cabin lights and avionics, as well as "bleed air" to run the air conditioning packs to keep the cabin cool while passengers are boarding at the gate. Furthermore, the APU provides the high-pressure air necessary to "crank" and start the main engines before departure. While you can't see the engine itself from the ground, you can often see shimmering heat haze or hear a high-pitched whine coming from that hole in the tail while the plane is parked. In the event of a total engine failure in flight, the APU can be started as an emergency backup to ensure the pilot still has electrical power and hydraulic pressure to control the aircraft.