A man takes pictures of a flight information board displaying the Scheduled Time of Arrival (STA) of Malaysia Airlines flight MH370 (top, in red) at the Beijing Capital International Airport in Beijing, March 8, 2014. Kim Kyung-Hoon/Reuters

Exactly one year after Malaysia Airlines Flight 370 disappeared without trace during a flight from Kuala Lumpur to Beijing, an independent investigative group released a 584-page report on Sunday detailing what's known about the missing plane.

Their conclusion? Not much.

If anything, the odds of finding an answer seem worse than before: The report found that the battery in an underwater locater beacon expired over a year before MH370 began its journey on March 8, 2014. Najib Razak, the Malaysian prime minister, sounded pessimistic.

"The lack of answers and definitive proof—such as aircraft wreckage—has made this more difficult to bear," he said on Sunday.

The mystery surrounding MH370 is almost unprecedented in aviation history. The closest recent comparison is Air France 447, an Airbus A330 that disappeared into the Atlantic Ocean minutes into a Rio de Janeiro to Paris flight on June 1, 2009. Even though investigators spotted debris and bodies in the days after the crash—giving them a clear idea of where the plane might have sunk—it took two years to locate the black boxes on the ocean floor.

The search for MH370, by contrast, is far more challenging. The first problem is the sheer lack of physical evidence: There are no seats, no luggage, no oil slicks, body parts, or anything else. Such a phenomenon isn't totally unheard of in aviation history: Most famously, no evidence was ever recovered from Amelia Earhart's doomed solo flight in 1937. But to lose an entire Boeing 777 in the hyper-connected, technologically advanced modern world is far more difficult to accept.

Unsurprisingly, the MH370 mystery has proven to be fertile ground for conspiracies. Some theorists have speculated that the plane landed safely on one of Indonesia's thousands of uninhabited islands, while others claim it was destroyed in an attempt to recover insurance money. More plausible are theories that the flight's experienced pilot, Zaharie Ahmad Shah, brought MH370 down in order to end his own life. Such suicide missions, though, are quite rare in aviation history, and Sunday's report found no evidence that Shah or his co-pilot Fariq Abdul Hamid suffered from depression.

Amid the frustration and bewilderment are the enormous logistical challenges of finding objects on the ocean floor. Investigators have covered 43 percent of roughly 60,000 square kilometers that comprise the main search area. Yet comparisons to land are only so useful: Human exploration and satellite imagery have brought back detailed examinations of even the remotest parts of the world. The sea, meanwhile, is a different story. Oceanographers have only mapped about 5 percent of the ocean's floor. Scientists know more about the surface of the moon.

The challenges of conducting the search have also taken a toll on the men and women tasked with finding the plane.

"Seven days' sail from nearest civilization, we're an awful long way from serious medical facilities," said Paul Kennedy, a project director on one of the four ships leading the search mission. "Essentially, it's like Antarctica."

Warren Truss, Australia's deputy prime minister, said he expected the search operation to be largely completed by May, and prime minister Tony Abbott remains optimistic that the mission will ultimately succeed. But the cost of the operation has imposed an additional limitation on the investigation.

"I cannot promise that the search will go with this intensity forever," he said.

For the friends and family of the missing, the mystery surrounding MH370 is a slow-burning nightmare made worse by the fact that it shows no sign of resolution. In January, the Malaysian government officially declared the event to be an "accident" and stated that the 239 missing are presumed dead. But not everyone is satisfied with this designation.

"There is no proof, no debris, not a single piece of evidence to tell us that they are really in the Indian Ocean," the wife of the the MH370 inflight supervisor, Patrick Gomes, told the Associated Press. "If this is really the end, show us something."

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