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A specially trained dolphin made an unusual discovery for the Navy during a mine-training exercise off the San Diego coast recently: a 130-year-old, self-propelled Howell torpedo.

A bottlenose dolphin named Ten made the find in March in waters not far from the Hotel del Coronado, according to various reports in the past few days, including the San Diego Union-Tribune and the Los Angeles Times. A week later, a second bottlenose dolphin named Spetz confirmed the sighting.

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"A diver went down and began to sweep the dust and debris off, and they realized that they had found something significant," Christian Harris, operations manager with the Navy Marine Mammal Program, told the Union-Tribune on Monday. "Subsequently, it was recovered, and that's when the light went on that what the dolphins had discovered was a really old marine artifact."

It remains unclear how the torpedo, which was missing its warhead, came to be sitting in waters off San Diego.

Fifty Howell torpedoes—named after its inventor, Lt. Cmdr. John A. Howell—were made from 1870 to 1889, and were used on Navy battleships and torpedo boats during that time. They were 11 feet long, powered by a 132-pound flywheel, and had a range of 400 yards with a speed of 25 knots.

Only one other Howell torpedo is known to exist, and it sits on display at the Naval Undersea Museum in Keyport, Washington.

The Navy has been training dolphins at its Point Loma facility since the 1960s. According to the Times, the Navy has 80 dolphins and 40 sea lions being trained for mine detection, mine clearing, and swimmer protection.

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The Union-Tribune explains how dolphins perform mine detection:

During training and the actual hunting of mines, a dolphin waits to receive a cue from its handler before it begins to search a specific area. Once the dolphin completes its search, it reports back to its handler, giving one response if a target object is detected and a different response if no target is found.

In this case, the dolphin touched the front of the boat to indicate a find, and the second dolphin did the same to confirm the discovery.

"We've never found anything like this," Mike Rothe, who heads the marine mamal program, told the Times. "Never."

The torpedo is being kept in a tank of water to prevent corrosion and is likely to be shipped to the Naval History and Heritage Command at the Washington Navy Yard, according to KPBS.

Logan Tittle of Newsy had this report about the odd discovery:

Photos are handouts from SPAWAR, the Space and Naval Warfare Systems Command. Dolphin photo courtesy of the Navy