The mystery surrounding Flight MH370 has spawned an industry of theories about its fate, particularly on the Internet.

Here's a look at some, including one that is out of this world - really.

NORTH KOREA TOOK IT

Some conspiracy theorists have asked if Pyongyang was the "missing link" in the investigation, saying the plane might have been hijacked at the order of North Korean leader Kim Jong Un so the country could access the Boeing 777's technology.

PUTIN STOLE IT

Former pilot and aviation expert Jeff Wise speculated that MH370 might have landed in Kazakhstan as part of a Russian-engineered plot, reported Reuters.

But he admitted he had "no idea" why Russian President Vladimir Putin would want to steal the plane. His theory is dependent on somebody on board MH370 tampering with key satellite transmission data to give the impression it flew south.

SHOT DOWN BY US-THAI MILITARY

The book Flight MH370 - The Mystery suggested that MH370 had been shot down accidentally during a joint US-Thai military exercise in the South China Sea.

Such accidents have happened before: Korean Air flight 007 was shot down by the Soviet Union in 1983 and the US Navy downed an Iranian airliner in 1988.

The book also claims that search and rescue efforts were sent in the wrong direction as part of a cover-up.

INSURANCE SCAM

In March 2014, Malaysian police refused to rule out the possibility that it may have been used in an elaborate insurance scam.

MH370 AND MH17 WERE SAME PLANE

This theory is based on photos of the MH17 crash site in Ukraine, which proponents argue show that the second crashed plane was in fact MH370, a later 777 model that had some small changes to its body.

How MH370 could have been hidden for six months before being substituted for MH17 - and what happened to the bodies - is not explained.

HIJACKED BY TERRORISTS

A retired US lieutenant- general spread the theory that the plane was flown to Taleban-controlled Pakistan, to be used to carry weapons of mass destruction for an attack on Israel.

ALIEN ABDUCTION

A poll conducted by CNN in 2014 found that 9 per cent of Americans believed that "space aliens or beings from another dimension were involved".

SUICIDE OR REVENGE ATTACK

Aviation expert David Learmount said: "There's only one thing it can be - a deliberate act by someone on board, probably the captain."