The Hunterian Museum and Art Gallery, University of Glasgow

The 60-foot-long Megalodon, the world’s largest known shark, died out 1.5 million years ago ... or did it? Some scientists think the monster shark is still among us, and unusual sightings have been claimed for many years.

The British survey ship HMS Challenger, while dredging a seabed near Tahiti in 1875, pulled up a pair of Megalodon teeth, one of which is shown here. Over the years, dating of the teeth has produced mixed results. Some dates fall in the 10,000-15,000-year-old range. That’s a far cry from 1.5 million years ago, when Megalodon supposedly went extinct.

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Naturalist David Stead, author of the book "Sharks and Rays of Australian Seas," wrote about “an immense shark of almost unbelievable proportions” that was seen by many fishermen in 1918. The men “refused to go to sea to their regular fishing grounds in the vicinity of Broughton Island, (New Zealand)” because they were too afraid of this gigantic shark, which Stead and others since have said could have been Megalodon.

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“Living fossil” sharks, such as the frilled shark seen here, have primitive characteristics and possible ancient ancestry. “Up until recently, it was supposed that, like most marine organisms, sharks suffered a catastrophic extinction at the K/T boundary (when dinosaurs went extinct),” David Ward of the Natural History Museum in London told Discovery News. “However, the studies that suggested this did not take into account the changes in sea level, climate and the amount of the sedimentary record missing.” He added that a study now in press, concerning the Danish site Stevns Klint, “demonstrates that there was very little extinction of deeper water sharks.” Megalodon could have fallen into that category.

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Many scientists believe that Megalodon resembled a stockier version of today’s great white sharks, such as the one in this image. It's possible that sightings of very large “great white sharks” could actually be Megalodon individuals.

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Discoveries of gigantic sea creatures once considered to be mythical continue into modern times. Giant squid can grow up to 30 feet in length and possibly more. These deep-sea dwellers tend to be the most elusive and therefore least well known. Megalodon could be another resident of the deep that has not yet been seen in modern times.

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Author B.C. Cartmell in his 1978 book "Let’s Go Fossil Shark Tooth Hunting," includes this account:

“In the 1960s along the outer edge of Australia’s Great Barrier Reef, an 85-foot ship experienced engine trouble, which forced it to weigh anchor for repairs. Although the men subsequently refused to openly report what they had seen for fear of public ridicule, the captain and his crew later told friends of sighting an immense shark as it moved slowly past their ship. Whitish in color, they were awed by its size. It was as long, if not longer, than their boat! Experienced men of the sea, they too were certain the creature was not a whale.” So what was it? Other writers, such as author Rick Emmer, suggest it could have been Megalodon.

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Novelist Zane Grey, famous for tales about the Wild West, such as “The Thundering Herd,” was also an avid deep-sea angler. Rick Emmer, in his book "Megalodon: Fact or Fiction?" writes that Grey described “what some people think might have been a live Megalodon.”

Grey wrote about what he called “one of the man-eating monsters of the South Pacific,” which left him “more frightened than I remember for a long time.” He then went on to describe a massive shark that was considerably longer than his 40-foot-long boat. He described it as being “yellow and green … (with a) square head, immense pectoral fins and a few white spots.” Grey shared that it was not a “harmless white shark,” but something else that he had never viewed before.

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The Sea of Cortez, also known as the Gulf of California, is home to many marine mammals, such as these dolphins. The marine mammals, in turn, tend to attract larger predators like sharks. Fishermen in the area have long reported seeing extremely large sharks. Locals even refer to a creature known as “Black Demon,” said to measure some 60 feet long. Fisherman Eric Mack, in July 2008, reported a boat strike with what could have been the Black Demon, which had a tail that rose five feet out of the water. This unknown species could perhaps be Megalodon.

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It remains unclear how Carcharodon megalodon went extinct, if it did. The supposed extinction of the huge shark, whose jaw and teeth have been reconstructed at the North Carolina Museum of Natural Sciences, remains a mystery. Did oceanic cooling and sea level drops do it in? Was there a decline in its food supply? Was it out-competed by new species? Or could it still be alive? Questions remain.

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The January 2014 issue of the journal Food Control will document how shark meat recently for sale in a Chinese food market located in Italy was labeled as being Megalodon. Shark meat, as this photo indicates, is still commonly sold in many markets around the world. Was the label “a nonsense scientific name … of an extinct species of shark,” as Priscilla D’Amico of the University of Pisa and her colleagues wrote? Even if the label was a mistake, the fishermen might have been on the right track. Hope persists that a relic population of Megalodon still exists.