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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 a comic book. Known as the mysterious "black box," these flight-data recorders are actually not black but orange — and when a plane falls from the sky, they're sometimes the only thing that can help authorities discover exactly what happened and why.

The June 1 Air France wreck in Brazil inspired the largest marine search for a black box in aviation history (which so far has turned up nothing), and now another sea crash has experts scanning the Indian Ocean for the flight recorders to the Yemenia Airbus A310 jet that went down near the Comoros Islands in the early morning hours of June 30.

In 1953, Australian scientist David Warren was investigating the crash of a De Havilland Comet in India. Warren couldn't determine the cause of the accident — in which the jet went down six minutes after takeoff, killing all 43 people onboard — because there wasn't any useful information preserved in the crash. Over the next few years, he developed a prototype for a flight-memory recorder that would track basic information like altitude and direction. Encased in asbestos and metal, the data and sound recorder was nicknamed the "black box," after the general term for a seemingly magical gadget that no one knows how to work.

Airlines were using black boxes by the end of the the 1950s, but the instruments didn't become a mandatory feature until 1960, when the Federal Aviation Administration required all commercial planes to carry them. Initial versions contained literal tape recorders and were about the size and shape of a basketball. After a number of black boxes were destroyed in crashes (the tapes melted in fire), they were moved in 1965 from their original position in the landing wells to the rear of the plane — the area most likely to survive an impact. That same year, they were also required to be painted orange or yellow to aid visibility.

These days, airplanes actually have two black boxes, the voice recorder and the flight data recorder. They can withstand temperatures up to 2,000°F and impact forces up to 100 Gs. (A G is equal to the force of the earth's gravity.) They track pilots' conversations, engine noises, air-traffic-control commands, fuel levels, landing-gear extension and retraction and dozens of other clicks and pops that might offer insights about a plane's final moments. The boxes are made out of quarter-inch-thick panels of stainless steel. And in case you're wondering, an entire airplane can't be made out of the same material or it would be too heavy to fly.

Since the 1960s, black boxes have recorded some astonishing things. In a 1990 incident, a pilot was sucked halfway out of a broken windshield on a British Airways flight; a flight attendant held on to his legs as the co-pilot landed the plane (the pilot survived). In 1994, an Aeroflot pilot allowed his 12-year-old daughter and 15-year-old son to play with the plane's controls during a Moscow-to–Hong Kong flight. "Can I turn [the wheel]?" the black box recorded the boy saying. "Turn it." The pilot replied. "Watch the ground as you turn. Let's go left." Moments later, the plane crashed into the Siberian wilderness, and all 75 people onboard died.

And of course, there was the black box of United Flight 93, which recorded 30 minutes of fearful struggle as passengers overpowered terrorist hijackers and crashed the plane into a Pennsylvania cornfield on the morning of Sept. 11, 2001. United 93's passenger voice recordings were the only tapes ever to be made available to victims' family members.

Both the Air France and Yemenia Airbus flights crashed into the ocean, which makes their black boxes incredibly hard to recover. The devices are built to withstand depths to well more than the 15,000 ft. in which Air France flight 447's boxes probably now find themselves. The boxes send out a homing signal, activated on impact, that lasts for 30 days. The time is pretty much up for Air France's beacons, but it's a good bet they'll turn up eventually; of the 20 airplanes that have crashed into water over the past 30 years, only one is known to have lost its black box forever. Even the South African Airlines Boeing 747 that went down between Taiwan and Johannesburg in 1987 had its voice and data recorders recovered from an ocean depth of 14,000 ft. And it took only 14 months.

• Claire Suddath
• It can withstand high temperatures and impact forces of 100 Gs. It reveals secrets of airplane crashes, including what went wrong and why. And it's not black at all