Submarine

A submarine or U-boat was a ship designed to operate underwater.

In 1492, the Fourth Doctor claimed that Leonardo da Vinci's submarine design wasn't practical. (TV: The Masque of Mandragora)

British subs
In the early 19th century, Sir Adrian Stapleford tried to develop a submarine for the Royal Navy. His assistant Sam and he nearly suffocated in it when Lord Trumpsworth sabotaged it. They were rescued by the Fourth Doctor, who convinced Adrian to discontinue work on the craft. (COMIC: The Emperor's Spy)

The first British submarine was launched in 1901, as was announced in the copy of The Times the Fifth Doctor found aboard the S.S. Shadow. (TV: Enlightenment)

In the 1970s, Captain John Hart sent out a submarine expedition led by Commander Ridgeway to search for the Sea Devils. The Sea Devils captured the submarine and its crew, but the Third Doctor helped free them. (TV: The Sea Devils)

Some time later, a group of Daleks based underneath Antarctica used a captured naval submarine to attack Sydney as part of their plan to use the city as a base from which to convert humans into slaves and conquer Earth. (COMIC: *Sub Zero)

Some time later, used time-displaced Scottish warriors to seize a nuclear submarine and threaten Britain with obliteration if he wasn't given the Doctor's TARDIS. (COMIC: The Glen of Sleeping)

In 2006, Mickey Smith launched Harpoon missile UGM-84A from the submarine HMS Taurean at 10 Downing Street. (TV: World War Three)

German U-boats
During World War I, a German U-boat, the U-20, sank the RMS Lusitania on 7 May 1915, which precipitated the entry into the United States of America into the First World War. (AUDIO: The Sirens of Time) In World War II, a German U-boat patrolling the English Channel in 1943 was sent forward in time to October 1964, where it threatened to torpedo the Isle of Wight ferry. Sir Toby Kinsella of the Intrusion Countermeasures Group ordered Group Captain Ian Gilmore and his squad to investigate and render assistance to the personnel of the nearby Lanchester Naval Base. (AUDIO: State of Emergency)

The 9th Doctor wore a U-boat captain's jacket from 1938. How he acquired up it is unknown.

At some point, Alec Palmer was involved in operations in the North Sea to disrupt U-boats. (TV: Hide)

Other human subs
In 1983, the frozen body of Grand Marshall Skaldak was picked up and revived in the Soviet submarine, the Firebird. (TV: Cold War)

In 1993 on a parallel world, Silurians used Kronosaurs to harass the remaining human submarines. (PROSE: Blood Heat)

In 2006, the Ninth Doctor, Rose Tyler and Jack Harkness investigated the decommissioned submarines at an abandoned Russian naval base. (PROSE: The Deviant Strain)

Extraterrestrial subs
In 2594, the SSSSSSS used a submarine to explore Tyler's Folly. (PROSE: Down)

On Marinus, the Voord used one-man glass submarines to get to the Conscience of Marinus. One of these submarines fractured and the acid sea killed the Voord within. (TV: "The Sea of Death")

On Olleril, Luminus was based on a submarine called the Gargantuan. (PROSE: Tragedy Day)