On March 8, Malaysia Airlines Flight 370 disappeared during its flight between Kuala Lumpur and Beijing. Three days later, the plane and its 239 passengers and crew members have yet to be found. How, in this modern world, can a plane so thoroughly and mysteriously fall off the radar and be so hard to find?

The most important fact to know, according to Wired, is that planes that have flown 100 to 150 miles offshore are no longer tracked by radar. Radar stations on land track planes for a while, but after that it's up to pilots and crews (and sometimes automated computers) to check in via high-frequency radio at predetermined times to report their location, speed and altitude. This isn't done all the times, though, and there are many gaps in the system during which a plane is out of communication.

Planes use GPS for their navigation, but that is a one-way information stream telling the pilots where the plane is located but not transmitting that data back to anyone else. Some planes carry tracking devices, but much like the same systems on your smartphone, "GPS and tracking devices only work if they're turned on or if they're not destroyed," Scott Hamilton, an aviation analyst, told NBC News.

Once the plane fell — for whatever reason — it did not crash on solid land. "The simple hard truth is it's very difficult to find things in the water," retired Marine Corps pilot and aviation consultant Col. J. Joseph told Wired. "It's very, very difficult to spot things in the water unless you're on top of it."

The search area is so broad that it's almost too much area to cover. Flight 370 probably went down over the South China Sea or the Gulf of Thailand, representing nearly 1.5 million square miles. The joint search-and-rescue effort is concentrating on the mostly likely 12,000 square miles of those ocean waters, but even that is a lot of territory to search.

There is also uncertainly about exactly where to look. Unconfirmed reports indicate the plane may have made a U-turn before it disappeared, meaning it could have ventured hundreds of miles off its original course, according to the Sydney Morning Herald. If Flight 370 wasn't on its normal flight pattern, that adds a new layer of complexity in terms of where to look and where it could have crashed.

The speed of flight also complicates matters. If the plane broke apart mid-air, the plane's momentum could have carried debris out over a great distance, experts told Scientific American. This would have been especially likely if the plane was still at a high altitude when it broke up. “If something catastrophic happened, that's seven miles up," Joseph told Wired. "Winds at that altitude are sometimes over 100 knots. Based on that wind, small pieces are going to be moved a lot of different places."

Experts seem to remain convinced the Flight 370 will be found. The only question that remains is when.

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Why can't we find Malaysia Airlines Flight 370?
Experts point to radar, which doesn't track planes that far out to sea, and the sheer vastness of the ocean.