The safety record of the Boeing 777 is nearly perfect, which makes the disappearance of Malaysia Airlines Flight MH370 near Vietnam especially mysterious to aircraft experts.

"This is so rare, it is absolutely baffling and it's baffling that we don't have any better answers this long after the mishap actually occurred," ABC News aviation consultant Col. Stephen Ganyard said. "So, lots of questions to be answered, [but] there's very little direct evidence, very few facts that we can go on."

Ganyard says the aircraft has a great safety record.

"We've seen a recent mishap in San Francisco, the Asiana crash, which involved a triple-seven, but in that case it's pretty clear that that was due to pilot error," he said.

An air search and rescue mission in the South China Sea, where the plane likely crashed, was under way over an area of 4,000 square miles.

The team is searching for the plane's black box, which records flight data and cockpit voices and can be detected through ultrasonic "pingers."

Even though the plane likely crashed in relatively shallow water, which makes these pingers work more effectively, underwater plane crashes are notoriously difficult to pinpoint.

The last flight to disappear like this was an Air France flight in 2009. It was two years before the black box was found and the cause of the crash - a combination of mechanical error and pilot oversight - determined.

Those causes will also be investigated in this disappearance, Ganyard said, as well as weather issues and even terrorism.

Airline officials lost contact with the plane, which was carrying 239 passengers, two hours into the Kuala Lumpur-to-Beijing flight, at 2:40 a.m. local time (18:40 GMT Friday).

The plane, which was carrying three Americans, went off radar without sending out a distress signal, meaning that whatever happened likely happened quickly.

The outcome is likely tragic, according to ABC News aviation analyst John Nance.

"If you have an airplane that comes out of 35,000 feet in an uncontrolled way or in pieces, the possibility of anybody surviving that kind of fall is very, very remote," Nance said. "But, again, we don't want to stomp on any possibilities, because miracles do occur."

The plane was due to land in Beijing at 6:30 am.