In an era when we've all got GPS in our pockets, OnStar in our cars and the NSA tracking anyone, anywhere, it is still possible–although rare–for an airliner to seemingly vanish.

That appears to be what happened to Malaysia Airlines Flight 370, which disappeared about an hour after leaving Kuala Lumpur for Beijing on Friday night. As of Monday, search and rescue teams from nine countries including the United States had not found any trace of the Boeing 777-200 or the 239 people aboard. There are many theories about what went wrong, but the airline, Boeing and investigators in Malaysia have so far refused to speculate or offer any insights.

Whatever happened, it happened quickly, aviation experts said, and catastrophically. The fact it happened over the ocean–presumably the South China Sea, but possibly the Gulf of Thailand–means it could be months or years before we know exactly what went wrong. The ocean is a very big place, and finding clues will be slow. It took investigators two years to recover the black box data recorder from Air France Flight 447, which went down over the Atlantic on June 1, 2009.

"The simple hard truth is it's very difficult to find things in the water," said retired Col. J. Joseph, a former Marine Corps pilot and aviation consultant.

The most chilling thing about this is the fact the plane seemingly vanished without a trace. The captain, who had more than 18,000 hours of flight time, gave no warning, issued no mayday. There was no indication anything was amiss. This is not terribly unusual, because a flight crew's first priority in an emergency is dealing with the situation at hand. "Aviate, navigate, then communicate" is the mantra. Airline pilot and blogger Patrick Smith says the radio silence "doesn't startle me."

"It's actually uncommon for there to be a distress message," he said. "It goes one of two ways. The first is something happens so catastrophically and so suddenly that there wasn't time for it. Secondly, crews are trained so that communicating with the ground is secondary to dealing with whatever urgency is at hand."

That might explain why Capt. Zaharie Ahmad Shaw and First Officer Fariq Ab.Hamid didn't tell air traffic control what was wrong, or issue a call for help. But how is it possible that air traffic control didn't know exactly where Flight 370 was when it went down?

Because it was over the ocean.

There's No Radar Tracking Airliners Over the Ocean ————————————————–

It is a misconception that airline pilots are in constant communication with air traffic control, or that planes are constantly watched on radar. Once a plane is more than 100 or 150 miles from shore, radar no longer works. It simply doesn't have the range. (The specific distance from shore varies with the type of radar, the weather, and other factors.) At that point, civilian aircraft communicate largely by high-frequency radio. The flight crew checks in at fixed "reporting points" along the way, providing the plane's position, air speed, and altitude. It isn't uncommon to maintain radio silence between reporting points because cruising at 35,000 feet is typically uneventful. Some aircraft communication systems don't require pilots call in; flight management computers transmit the info via satellite link.

Although modern flight management systems use GPS for navigation, that only tells the airplane where it is–it does not tell air traffic control where the plane is. It's a bit like taking your iPhone into the heart of the Mojave desert: Your GPS will tell you where you are, but you can't use Find My Phone because there's no cell coverage. Although it would be possible to stream data from an aircraft in real time via satellite, implementing such a system across the industry would cost billions of dollars, Smith said.

Still, it is highly unlikely that Flight 370 went down silently, Joseph said. Many commercial aircraft have an emergency locator beacon that the flight crew can trigger in an instant. It also will activate under certain circumstances, such as impact with water–though it isn't effective a great depths. And although civil aviation systems don't have radar or other tracking technology at sea, military and security agencies almost certainly do. It's possible a government ship, airplane or satellite captured some clues, as was the case when a Soviet fighter jet brought down Korean Air Lines Flight 007 in 1983.

"I would be very surprised if, on somebody's radar data, this event was not recorded," Joseph said.

Other data streams might provide insight. Reuters reported Flight 370 was equipped with ACARS, a maintenance computer capable of transmitting data to the airline, alerting mechanics of needed repairs or adjustments. Although the system does not typically transmit data in real time, it does send info periodically during a flight. Such data can provide clues, but only after the fact. ACARS helped investigators determine why Air France Flight 447 crashed. Although Boeing offers a more advanced system, called Airplane Health Management, that provides real-time troubleshooting and monitoring of flights, Flight 370 did not use it.

The Debris Field Is Big, And the Ocean Is Bigger ————————————————

Still, if a plane goes down, it's gotta land somewhere, which means there should be something out there. But after three days of searching, investigators still hadn't found any sign of the plane. This is unusual, but not unprecedented.

The most obvious explanation is search and rescue vessels aren't looking in the right place because they aren't sure where the plane went down, Smith said. It's also possible, though highly unlikely, that the plane remained largely intact after hitting the water and sank.

There's been a lot of speculation about what might have happened, and airliners have been brought down by everything from an onboard fire to an intentional crash by the pilot and, of course, terrorism. A catastrophic failure of the airplane–the failure or loss of an essential component, for example, or explosive decompression–is another possibility.

Aviation experts said it is far too soon, and too little is known, to speculate on what might have happened. But many agree that whatever happened was sudden and almost certainly occurred at high altitude, scattering the debris over a vast area.

"If something catastrophic happened, that's seven miles up," Joseph said. "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."

Any aerodynamic pieces–wing sections, say, or pieces of the tail–will be blown around like a bag in the wind. Heavier pieces like an engine or landing gear will fall straight down. Fuel and other fluids will be scattered, leaving little evidence below. This is what happened when the space shuttle Columbia disintegrated during reentry on February 1, 2003. The difference there was the disaster occurred over land. Spotting debris on open ocean is much, much harder.

"It's very very difficult to spot things in the water unless you're on top of it," Joseph said.