The Kodiak Dinosaur

In 1969, the shrimping boat Mylark was fitted with a top-of-the-line electronic detector to map the ocean floor in Raspberry Straight, off Kodiak Island, Alaska.  During the trip, an operator noticed a strange shape about 330 feet down--a 200-foot long object, shaped like some sort of dinosaur, that appeared to be swimming.  The experienced fisherman who were operating the device swore it bore no resemblance to any kind of whale or any other sea creature known to inhabit the area.  The operators captured an image of the reading, but it can no longer be found.

Source: Unknown Explorers