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(Image: SMITHSONIAN CHANNEL)

Filmmaker Dave Riggs was given the job of tagging sharks along Australia's coast, and was blown away by the fearlessness of one in particular.

The enormous creature – dubbed Shark Alpha – barged Riggs' boat as the crew placed a tag on its side, before disappearing into the depths of the ocean.

Four months later the tag was found on a beach two-and-a-half miles away – and what it's data revealed would spark a mystery that continues today.

On Christmas Eve, 2003, the great white plunged at high speed 1900ft down into the vanishing blackness where something very strange happened.

Its temperature rocketed from 46 to 78 degrees fahrenheit – a heat the shark could only reach in the belly of another animal.

For eight days the tag stayed at that temperature, moving between the surface and a depth of 300ft before being belched up.

"When I was first told about the data that came back on the tag that was on the shark I was absolutely blown away," said Riggs.

(Image: SMITHSONIAN CHANNEL)

* * MONSTER OF THE DEEP: VIDEO CLAIMS TO SHOW 60ft SHARK * *

The question that not only came to my mind but everyone's mind who was involved was, 'what did that?' It was obviously eaten.

He asked: "What's going to eat a shark that big? What could kill a three-metre great white?"

Well nobody knows for sure, but there are some theories.

Riggs told a 2014 documentary that the likeliest answer was an even bigger shark.

"The internal temperature of the animal that ate the shark is a weird one," he told the Smithsonian Channel.

"It appears to be too low for a killer whale and too high for another shark, unless it was massive."

(Image: SMITHSONIAN CHANNEL)

It's the same theory reached by YouTube channel Animalist, which gave an idea of its size.

"According to researchers the supposed colossal cannibal great white shark would have been 16ft long and over two tons," said one video.

"No one is quite sure why this happened but experts suspect that it may have been the result of a territorial dispute or extreme hunger."

But this claim has been reject by others, with some saying the shark was perhaps devoured by a killer whale.

Camrin Braun of the Woods Hole Oceanographic Institution in Massachusetts, USA, said the temperatures didn't add up.

"White shark stomach temperatures are thought to be on the order of 65 to 70F – maybe even less at great depth in cold water," he told National Geographic in 2014.

"Thus, it seems likely that a feeding killer whale that is ingesting cold seawater and food could easily have a stomach temperature of 78F."