The disappearance this past weekend of Malaysia Airlines Flight MH370, carrying 239 people, remains a mystery for now. Despite a search effort involving ships and aircraft from multiple countries, all potential leads as to the fate of the craft—including a long oil slick and a floating object thought to be a door of the plane—have failed to pan out.

How does a jetliner simply disappear?

Conventional tracking systems performed well; civilian and military radar tracked the plane until it vanished. As Patrick Smith, a pilot and creator of a blog called “Ask the Pilot,” points out, “They knew where the aircraft was up until the point where something catastrophic happened.” The plane followed the path that the same flight had taken a few days earlier, according to the private flight-tracking Web site flightradar24. So the next question is why evidence of the plane has yet to turn up. Smith says this is mostly due to the size of the oceanic search area. He’s confident that the wreckage eventually will be found.

There are a variety of methods for tracking planes in the air, says Sid McGuirk, associate professor and air traffic control specialist at Embry–Riddle Aeronautical University’s Daytona Beach College of Aviation. At the airport until about a minute after takeoff, he says, “the tower” is in charge, with flight controllers visually tracking planes. As soon as the plane is out of sight, radar systems take over. Radar works well on land, where coverage is extensive, but about 300 kilometers from shore the signal becomes too weak, McGuirk says. In 2009, when Air France Flight 447 crashed in the Atlantic while en route to Paris from Rio de Janeiro, it was no longer being tracked by radar, which is what made finding its wreckage so difficult: more than five days for the first debris and two years before the black-box flight recorders were located at a great depth on the ocean floor.

Unlike the Air France flight, however, Flight MH370 was still within the range of radar tracking systems. Flightradar24, which does not use radar but instead a network of antennas that receive location information broadcast by planes via a technology called automatic dependent surveillance–broadcast, or ADS–B, tracked the plane to around the same spot. But radar and ADS–B also have limits. Flightradar24 loses ADS–B coverage below about 9,000 meters. And radar also can only track aircraft above a certain elevation, McGuirk says, although that can vary based on the proximity of the plane to the closest radar antenna.

A number of factors can contribute to where plane wreckage ends up. “Dynamics of aircraft can vary,” he says, including how they hit the ocean or whether they disintegrate in flight. In addition, “the aircraft’s flying pretty fast,” says Larry Cornman, a scientist at the National Center for Atmospheric Research. “It gives kind of a large window…for where the aircraft could be,” he says, “especially if it broke apart in flight.”

McGuirk thinks that the possibility of the plane being hijacked but still intact is unlikely; it would have required not only turning off the ADS–B system and the plane’s radar transponder, but also flying the plane below where radar could detect it. Then hijackers would have to land the plane unnoticed—a difficult task, given the size of a Boeing 777. “That’s sort of a far-fetched solution,” McGuirk says. “It’s not far-fetched to jump to the conclusion that some catastrophic event occurred onboard,” he says, especially considering the lack of emergency communications from the crew before the disappearance.

After the Air France disaster, which killed all 228 people onboard, some people called for systems that automatically beam an airplane’s location to satellites. It’s unclear, however, if such a system would have helped in the case of Flight MH370; the plane’s location was known before it disappeared. Cornman also points out that because commercial airplane crashes are so rare (with only 0.34 fatal accident per million departures 2002 through 2011, according to Boeing) the cost-benefit argument doesn’t favor adding these systems.

Ultimately, McGuirk says, we likely won’t know for awhile what happened to Flight MH370. “Anything that’s said right now is pure speculation,” he says. Right now, it’s “way, way too early in the investigation to come to any conclusions.”