One hundred years ago (in 1918) the USS Cyclops, an American World War I Navy cargo/troop ship hailed as a “floating coal mine,” should have docked in Baltimore, after a return run from Brazil, with a stop off in Barbados.
Reported to be the Navy’s biggest and fastest refueling ship at the time – the 540-foot long by 65-foot wide steel-hulled ship, with 309 men on board never arrived as scheduled on March 13, 1918, to her Chesapeake Bay berth, and its whereabouts still remain unknown.
Outfitted with 50-caliber machine guns to transport doctors and medical supplies to our American Expeditionary Forces in France during WWl, she was last seen after a 9 day load up of 10,000 tons of manganese ore in Barbados on March 4, 1918.– a denser but 2,500 ton lighter tonnage load  than her usual 12,500 ton coal cargo. 
Photo #: NH 76012  USS South Carolina (Battleship # 26) and USS Cyclops (Fuel Ship # 4)  Experimental coaling at sea while under way in April 1914. Rigging between the two ships was used to transfer two 800-pound bags of coal at a time. The bags were landed on a platform in front of the battleship's forward 12-inch gun turret, and then carried to the bunkers. Original photo is printed on AZO postcard, inscribed on the reverse: This is a picture of us coaling at sea last April. I have put a cross over where I stood. I unhooked bags of coal when they came over. It is raining when this picture was taken. We were out of sight of land off coast of Virginia. The donor, a seaman in South Carolina at the time, comments: it showed that this was possible but a very slow method of refueling. Nothing was heard of the test afterwards. For a view of the card's reverse side, with the quoted inscription, see: Photo # NH 76012-A.  Donation of Earle F. Brookins, 1972.  U.S. Naval History and Heritage Command Photograph.
The USS Cyclops, in the background, transferring bags of coal with the USS South Carolina in 1914.  (US Naval History and Heritage Command)
Built in Philadelphia in 1910, the USS Cyclops was load rated for 12,500 tons of coal and could lift two tons of it in single buckets along cables that ran the length of the ship, after being 30 days late, a headline in the New York Times on April 15, 1918 said "COLLIER  CYCLOPS OVERDUE BY A MONTH', next to a list naming the missing 309 Navy crewmen on board.
Numerous ships tried to locate the collier as she was thought to have possibly been sunk by a German submarine, but, there was no distress call, and nothing from the ship has ever been found (no wreckage, oil slicks or debris), speculation has raged involving the Bermuda Triangle, giant sea creatures and mutinies.
Two months overdue,  in May of 1918, Franklin D. Roosevelt, who then was an Assistant Navy Secretary, announced the Cyclops and all of its crew were presumed lost at sea, resulting in what still remains today, both, the largest loss of life in Navy history (unrelated to combat), and the US Navies only unexplained missing ship.
Marvin Barrash, a great nephew of one of the firemen on the Cyclops, has spent more than a decade researching the ship and believes it could be sitting in the deepest part of the Atlantic Ocean (the Puerto Rico Trench), which extends more than 27,000 feet below the surface.