CNN) -- A Boeing 777, one of the world's most reliable types of airliners, is missing, and no one knows why. Was it a

bomb? Mechanical failure? A hijacking gone awry? Pilots and others in the aviation community are deeply disturbed by the mystery surrounding Malaysia Airlines Flight 370.
It disappeared Saturday en route from Kuala Lumpur to Beijing over the Gulf of Thailand, somewhere between Malaysia and Vietnam. It's hard to believe that such huge questions remain four days after the Boeing 777-200ER went missing, carrying 227 passengers and 12 crew members.
A revelation Tuesday set off a fresh wave of speculation. Radar tracking detected the plane hundreds of miles off course, traveling in the opposite direction from its original destination, a senior Malaysian Air Force official told CNN. The aircraft had stopped sending identifying transponder codes before it disappeared, the official said.
The new details marked a dramatic twist in the investigation. But some reports have quoted other Malaysian authorities offering conflicting information, with some denying the military official's claim. What can cause an airplane to disappear? Search underway for missing Flight 370 Authorities 'puzzled' by missing flight Search area for missing plane widens
The situation is so unprecedented that experts have been careful in speculating about possible explanations.
Here are four scenarios they're talking about, and the related facts:
1. Scenario: Mechanical failure?
Fact: The absence of a debris field suggests the possibility that pilots were forced to ditch the plane and it landed on water without breaking up, finally sinking to the ocean floor.
Analysis: But if that were the case, then why no emergency signal? These planes are able to perform a "miracle on the Hudson" maneuver. They have the ability to glide more than 100 miles and belly land on the water with