KUALA LUMPUR – More than ten years ago, Malaysia Airlines Flight 370 disappeared into thin air, leaving behind one of the most perplexing mysteries in aviation history.

Despite extensive international efforts to uncover its fate, investigators remain in the dark about the details of the plane’s disappearance and the fate of its 239 passengers and crew. The search has been exhaustive, yet the questions only seem to multiply with time.

In a renewed bid for answers, the Malaysian government announced on Wednesday that a new search mission has commenced. A dedicated vessel is now scouring the vast expanses of the ocean, rekindling hopes that the elusive aircraft could, at long last, be located.

The previous extensive search efforts concentrated in the southern Indian Ocean, the presumed final resting place of the jetliner, yielded minimal results. Aside from a few small fragments that eventually drifted to shore, there has been no recovery of bodies or significant pieces of wreckage.

As the world watches with bated breath, here’s what is crucial to understand about this tragic aviation enigma.

Flight goes missing

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

“Good night, Malaysian Three Seven Zero,” the pilot said in the last radio call to Kuala Lumpur and the final communication before the plane crossed into Vietnamese airspace and failed to check in with controllers there.

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.

Among those aboard were two young Iranians traveling on stolen passports, a group of Chinese calligraphy artists, 20 employees of U.S. tech firm Freescale Semiconductor, a stunt double for actor Jet Li and several families with young children.

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.

The search was suspended in January 2017.

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.

The hunt is renewed

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.

It is unclear if Ocean Infinity has new evidence of the plane’s location. The company has said it would utilize new technology and has worked with many experts to analyze data and narrow the search area to the most likely site.