You never see them, but they're with you every time you fly. They record where you're going, how fast you're traveling and whether everything on your airplane is functioning normally. Their ability to withstand almost any disaster makes them seem like something out of the comic book. They're known as the black box. When planes fall from the sky, as a Yemeni airliner did on its way to the Comoros Islands in the Indian Ocean June 30, 2009, the black box is the best bet for identifying what went wrong. So when a French submarine(潜水艇)detected the device's homing signal five days later, the discovery marked a huge step toward determining the cause of a tragedy in which 152 passengers were killed. In 1958, Australian scientist David Warren developed a flight-memory recorder that would track basic information lke altitude and direction. That was the first model for a black box, which became a requirement on all US commerical flights by 1960. Early models often failed to withstand crashes, however, so in 1965 the device was completely redesigned and moved to the rear of the plane---the area least subject to impact---from its original position in the landing wells(起落舱架. That same year, the Federal Aviation Authority required that the boxes, which were never actually black, be painted orange or yellow to aid visiblility. Modern airplanes have two black boxes: a voice recorder, which tracks pilots conversations, and a flight-data recorder, which monitors fuel levelsengine noises and other operating functions that help investigators reconstruct the aireraft's final moments, Placed in an insulated (隔绝)case andurrounded by quarter inch thick panels of stainless steel, the boxes can withstand massive force and temperatures up to 2 000'F. When submergedthey re also able to emit signals from depths of 20 000 ft. Experts believe theboxes from Air France Flight 447, which crashed near Brazil on June I2009, are in water nearly that deep, but statisties say they re still likely to turn up. In the approximately 20 deep-sea crashes over the past 30 years, only one plane's black boxes were never recovered.