Over the years, the airline industry across the world has witnessed several tragic plane crashes, some of which have perplexed both aviation experts and the public.

From Amelia Earhart's ill-fated flight to the unsolved mystery behind the disappearance of Malaysia Airline Flight MH370, aviation authorities have faced several challenges over the years despite an improvement in flight tracking systems and specialized technologies to track pings from black boxes.

Here are few unsolved airline mysteries — in no particular order — that kept investigators clueless for years.

Amelia Earhart: The hunt for Amelia Earhart’s long-lost plane never really found a closure as the plane carrying the aviation pioneer and her navigator, Fred Noonan, went missing on June 1, 1937, as the two left   Miami, Florida in an attempt to circumnavigate the globe. It is believed that Earhart’s twin-engine Lockheed Electra crash-landed in the Pacific Ocean somewhere near their target destination of Howland Island. 

The International Group for Historic Aircraft Recovery (TIGHAR) has made the most significant strides in the search trying to make sense of Earhart’s mysterious disappearance. The group has been combing a remote South Pacific island called Nikumaroro since 1989, which many experts believe could be the resting point of Earhart and Noonan's plane. Some of the items collected by the group include improvised tools, shoe remnants and aircraft wreckage that experts believe are consistent with Earhart’s Electra.

The most compelling development in the case was made in 2014, when searchers reportedly identified a piece of the missing aircraft near an atoll in the southwestern Pacific Ocean, making it the first time a piece of wreckage found had been recognized as a fragment from Earhart’s plane, according to Discovery. TIGHAR linked the fragment with a patch of metal that was installed on Earhart’s plane. The government reportedly spent more than $4 million looking for Earhart. In January of 1939, Earhart was officially declared dead.

Flying Tiger Line Flight 739: This was a Lockheed L-1049 Super Constellation propliner chartered by the U.S. military that disappeared on March 16, 1962, over the Western Pacific Ocean. The aircraft was transporting 93 U.S. soldiers and 3 South Vietnamese from Travis Air Force Base, California to Saigon, Vietnam. There were 11 crew members on board the plane. An eight-day search for the missing plane yielded no concrete clues as to the whereabouts of the jet or the people on board, following which the search operations were called off. A total of 48 aircraft and 8 sea vessels scoured over 200,000 square miles of the ocean to locate the jet. 

Flight MH370: On March 8, 2014, Malaysian Airline Flight MH370, flying from Kuala Lumpur to Beijing, went missing with 239 people on board. Just hours after the plane lost contact with air traffic control, a search operation was launched to find the plane. Little was known at the time that the incident would end up becoming one of the greatest mysteries of aviation history. The plane went off the radar while traveling over the southern Indian Ocean. Investigators concluded that the plane may have crashed in a remote part of the southern Indian Ocean. Despite efforts that continued for more than two years, no trace of the plane or the people on board has been found till date.

mh370 update A Chinese vessel will join the ongoing search for Malaysia Airlines Flight MH370 next month. This photo shows French maritime gendarmes looking at a map indicating measures undertaken in the search for wreckage from the missing MH370 plane at the marina of Saint-Marie on the French island of La Reunion, Aug. 14, 2015. Photo: Getty Images/RICHARD BOUHET/AFP

The disappearance of Flight MH370 has now become the costliest search in aviation history with authorities spending an estimated $150 million. Malaysia, China, and Australia conducted the search for the plane but the hunt was called off earlier this year. While authorities of the tripartite nation faced criticism from the families of those on board the jet, Australia’s Transport Minister Darren Chester said that the cost of the search was not the reason for calling off the search. 

While the mystery behind what happened to Flight MH370 continues to linger, there have been several conspiracy theories that made the round just after the jet went missing, including claims of a cockpit fire, of a rogue pilot, and also hijack.

Bermuda Triangle Incidents: Over the past centuries, planes have disappeared or met with fatal accidents in the triangular area on the Atlantic Ocean known as Bermuda Triangle. The most shocking incident of all was in 1945 when five training flights that took off from Florida naval base disappeared while flying over the Bermuda Triangle. Despite search efforts, the planes and those on board were never found. In fact, a Martin Mariner flying boat that was sent for the search operation also went missing. 

Many experts have tried to solve the mystery as to while planes and ships disappeared while traveling through the area. However, the reason behind the disappearances still remains unknown with only conspiracy theories, including electronic fog engulfing an aircraft, hurricanes destroying aircraft,  or just simple human error.