With search crews just months away from finishing their thus-far fruitless sweep of a remote stretch of seabed where Flight 370 is believed to have crashed, officials appear no closer to solving one of the most mind-boggling mysteries of modern times. That stubborn lack of resolution has only increased speculation about what might have happened to the Boeing 777 after it vanished with 239 people on board on March 8, 2014. Some believe officials are simply looking in the wrong part of the Indian Ocean, while social media sites are peppered with comments suggesting they're looking on the wrong planet: "MH370 was abducted by aliens," reads a typical tweet.

"We knew this was a very high-profile, publicized event and because it was such a great mystery, there was going to be a lot of scrutiny," says the Australian Transport Safety Bureau's chief commissioner Martin Dolan, who is leading the search for the plane far off Australia's west coast. "We are always open to informed criticism. What we find a bit more difficult is when occasionally people criticize us on the basis of a misunderstanding or a misrepresentation of what we're doing or saying."

Here's a look at a few of the theories that investigators have considered but view as unlikely:

The plane went north instead of south

After veering off-course shortly after takeoff on its flight from Kuala Lumpur to Beijing, officials believe the plane flew south on a straight path into the abyss of the southern Indian Ocean. They arrived at that conclusion after analyzing exchanges between the plane's engine and a satellite. But some people insist the plane instead flew north into Asia, and that the satellite data indicating otherwise was tampered with.

Dolan dismisses that theory, noting that British satellite company Inmarsat, which provided the satellite data to investigators, is a widely respected company with a solid track record. There's no reason to doubt their data, he says.

"Those sorts of theories just seem to over-complicate what's going on here," Dolan says. "We think that had any data been manipulated, there would have been a trace of it."

Beyond that, a wing part from the plane washed ashore on Reunion Island in the western Indian Ocean in July, effectively eliminating the possibility that the rest of the plane ended up in the Northern Hemisphere. That said, a few people have suggested the wing flap was planted on the island by terrorists.

It went west to the Maldives

Some argue the plane must have traveled west to the remote Indian Ocean island nation of the Maldives after early reports emerged of locals spotting a low-flying plane in the area around the time Flight 370 vanished.

The military in the Maldives told Malaysia that those reports of sightings turned out to be false. Last year, Malaysian investigators traveled to the Maldives to examine possible debris that had washed ashore, but it was determined to be unrelated to Flight 370.

Former Australian Transport Minister Warren Truss has said that while the plane may have had enough fuel to reach the islands, it wasn't detected by air traffic control or any other local authority. The flight path to the Maldives is also inconsistent with investigators' satellite and radar data.

"It is not considered a likely possibility," Truss said last year.

It was shot down

One of the earliest theories suggested the plane was headed toward Diego Garcia, a British atoll in the Indian Ocean where the U.S. has a military base. The former head of the now-defunct Proteus Airlines, Marc Dugain, voiced his own theory that US military, fearing a Sept. 11-style attack, may have shot down the plane as it approached the atoll. The US has denied the aircraft came anywhere near Diego Garcia.

It was hijacked by passengers

Immediately after the plane disappeared, many speculated that one or more passengers hijacked the plane. This theory gained traction after it was discovered that two Iranians on board were traveling on stolen passports. Investigators cleared the two after finding nothing linking them to terror groups; it is believed they were trying to illegally immigrate to Europe. Police scrutinized the backgrounds of every passenger on the plane — but nothing suspicious was found.

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