A still from the film Shark Attack 3: Megalodon // A still from the film Shark Attack 3: Megalodon (Alamy)

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A still from the film Shark Attack 3: Megalodon

Could an unknown, 30-metre monster shark with seven-inch-long razor sharp teeth be lurking in the depths of our oceans? Australian fishermen swore they encountered such a beast near Borough Island, and a slightly smaller, 15-metre specimen was spotted in French Polynesia early last century.