Researchers are trying to find out what could have eaten a 3m/9ft shark they were following.
A tracking device, which washed up on a beach in Australia, showed that the healthy female shark had suddenly plunged to a depth of 580m/1,900ft at high speed. The tag also recorded a spike in temperature from 7°C to 25°C/46-78°F.
One theory for this is that the shark was eaten by an even bigger shark — the heat resulting from the larger predator's digestive system when the smaller shark was swallowed, before the bigger animal then took a dive to the depths.
Scientists now wonder if there is a "colossal cannibal Great white shark" out there somewhere. Others think a whale may have been responsible, while Internet users blame Kraken — and even Godzilla!
The quest to solve the mystery is the subject of a film, called Hunt for the Super Predator, which is due to air on the Smithsonian Channel on June 25.
You can watch a trailer for the documentary below:
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