What to know about the mystery of Malaysia Airlines Flight 370 as the search resumes

More than a decade ago, Malaysia Airlines Flight 370 vanished without a trace, sparking one of aviation's most baffling mysteries.

Despite years of multinational searches, investigators still do not know exactly what happened to the plane or its 239 passengers and crew.

On Wednesday, the Malaysian government said a vessel began a new search operation for the missing plane, reigniting hopes the aircraft might finally be found.

A previous, massive search in the southern Indian Ocean, where the jet is believed to have gone down, turned up almost nothing. Apart from a few small fragments that washed ashore, no bodies or large wreckage have ever been recovered.

Here is what to know about the deadly aviation tragedy.

Flight goes missing

The Boeing 777 disappeared from air-traffic radar 39 minutes after departing Kuala Lumpur for Beijing on March 8, 2014.

Minutes later, the aircraft's transponder stopped broadcasting its location. Military radar showed the jet turn back over the Andaman Sea. Satellite data suggested it continued flying for hours, possibly until fuel exhaustion, before crashing into a remote section of the southern Indian Ocean.

Theories about what happened range from hijacking to cabin depressurization or power failure. There was no distress call, ransom demand, evidence of technical failure or severe weather.

Malaysian investigators in 2018 cleared the passengers and crew but did not rule out ''unlawful interference.'' Authorities have said someone deliberately severed communications and diverted the plane.

The passengers came from around the world

MH370 carried 12 crew members and 227 passengers, including five young children. Most passengers were Chinese, but there also were citizens of the United States, Indonesia, France, Russia and elsewhere.

Many families lost multiple members.

The search covered a vast area

Search operations began in the South China Sea between Malaysia and Vietnam before expanding to the Andaman Sea and the southern Indian Ocean.

Australia, Malaysia and China coordinated the largest underwater search in history, covering roughly 120,000 square kilometers (46,000 square miles) of seabed off western Australia. Aircraft, vessels equipped with sonar and robotic submarines scoured the ocean for signs of the plane.

Signals thought to be from the plane's black box turned out to be from other sources and no wreckage was found. The first confirmed debris was a wing fragment, known as a flaperon, discovered on remote Réunion Island in July 2015, with additional fragments later found along the east coast of Africa.

In 2018, U.S. marine robotics company Ocean Infinity resumed the hunt, under a ''no-find, no-fee'' agreement, focusing on areas identified through debris drift studies. The effort ended without success.

The search faced enormous challenges

One reason why such an extensive search failed to turn up clues is that no one knows exactly where to look.

The Indian Ocean is the world's third largest and the search was conducted in a difficult area where searchers encountered bad weather and average depths of around 4 kilometers (2.5 miles).

It's not common for planes to disappear in the deep sea, but when they do remains can be very hard to locate. Over the past 50 years, dozens of planes have vanished, according to the Aviation Safety Network.

Malaysia's government gave the green light in March for another ''no-find, no-fee'' contract with Ocean Infinity to resume the seabed search operation at a new site stretching over 15,000 square kilometers (5,800 square miles) of water. Ocean Infinity will be paid $70 million only if wreckage is discovered.

However, the search was suspended in April due to bad weather. The government said Wednesday that Ocean Infinity will resume the search intermittently from Dec. 30 for 55 days in targeted areas believed to have the highest likelihood of finding the missing aircraft.