|Launched:||3 May 2008|
|Length:||17.76 m (58 ft 3 in)|
|Beam:||2 m (6 ft 7 in)|
|Speed:||6 knots (11 km/h; 6.9 mph)|
UC3 Nautilus is a privately built Danish midget submarine. It was launched on 3 May 2008 in Copenhagen, Denmark. It was built over a three-year period as an art project by Peter Madsen and a group of volunteers, and cost approximately US$200,000 to build (1.5 million DKK).
On launch day, the submarine displaced only 32 tons and was still incomplete. Nautilus was named at a well-attended ceremony on the west side of Refshaleøen and towed to the floating installation art barge ILLUTRON (formerly M/S Half Machine) in the Copenhagen South Harbor where further outfitting and installation of equipment happened over the next several months.
By August 2008, Nautilus could sail on its own, as steering and the main diesel engine drivetrain was completed. On the maiden voyage the submarine was sailed by a crew almost all from the now-defunct  Royal Danish Navy submarine division. The objective was to sail the submarine and evaluate her maneuverability, and the appraisal was favorable. In October 2008 Nautilus was submerged for the first time.
Nautilus can be crewed by up to eight people for surface operation and four when diving. Nautilus moves at five to six knots, depending on the weather and whether it is up on the surface or submerged.
The boat has two 1,500-liter tanks with fresh water and fuel. Trimming can be carried out with an electric pump, which is normal procedure, or with compressed air as fall-back, if the trim-pump fails. The main ballast tanks, of 8,000 liters, are always filled and emptied with compressed air, and can flush up to 400 liters of water in/out per second. Nautilus can go from sailing on the surface, to periscope depth in approximately 20 seconds. The boat has a theoretical crush depth of between 400–500 m (1,300–1,600 ft), but, as a precaution, the submarine only nominally dives to 100 m (330 ft).
There are no torpedo tubes or any other forms of armaments, but Nautilus has 16 portholes (8 on each side, 2 oversized) for direct observation. The periscope has 5 video cameras, providing 360 degree panoramic vision on video screens in the forward control room (beneath the deck hatch.) The Engine room has two diesel engines, only one of which is for direct propulsion. The other drives a 3-phase electric generator providing power for the onboard air compressor and battery charger. There are more than a tonne of large 12 volt batteries on board (aft in engine compartment,) supplying the electric DC engine which can be operated alone or in tandem with the main diesel engine on a chain-coupled drivetrain for turning the 80 kilogram, five-bladed brass propeller. Joystick-controlled electric servos operate the rudder (aft) and hydroplanes (front).
Nautilus is almost half the size of the Danish Tumleren class submarine. Almost half as long, half as wide, moving at about half the speed, and is staffed by only around half as many crew members. In the spring of 2009 an automatic snorkel system was installed, so the submarine could sail submerged on diesel power, and in August 2009 the submarine sailed out from the Port of Køge submerged.
Nautilus can be operated by a single person from the control room. All controls and indicators for buoyancy, pumps, engines, air pressure, communication, video and other electric systems are accessible from the captain's seat. However, as of July 2010 crew in Engine room still need to perform manual changeover from surface to dive-ready configuration, setting valves correctly for snorkel and diesel engine exhaust.
Nautilus can function on her diesel engine for up to eight minutes underwater but since the diesel engine, unlike the electric motor, requires a steady supply of air to operate using the diesel engine while submerged air pressure drops to close to that at 10,000 feet above sea level as the engine ingests air from the crew compartment unless the snorkel is used.
On a 2009 trip around Refshaleøen, Copenhagen, the submarine was visited by a group of submarine fans from Subsim, that for a day swapped the computer simulated submarine operations, for real submarine operations. Game developers from Ubisoft were also on board to find inspiration for their submarine computer game, Silent Hunter 5. They wanted to operate valves and ballast systems themselves.
Today, Nautilus is used by her builders and various others for recreation and expeditions.
One such expedition is the attempted launch of the rocket and spacecraft HEAT1X-TYCHO BRAHE built by Danish non-profit rocket group Copenhagen Suborbitals. On Tuesday 31 August 2010, the UC3 Nautilus pushed the launch platform Sputnik carrying the rocket and spacecraft from Copenhagen towards the launch area near Nexø, Bornholm.
In January 2011 Nautilus returned to Refshaleøen and will be put on shore for upgrades and an overhaul expected to last several months.
On the 28th of April 2017 Nautilus was launched again after thorough repairs and upgrades. 
- "Verdens største privatbyggede ubåd søsættes i København ("The world's largest privately built submarine launched in Copenhagen")". Jyllandsposten (in Danish). Retrieved 6 July 2010.
- "Klar til dykning ("Ready for diving")". Berlingske Tidende (in Danish). Retrieved 6 July 2010.
- "Tumleren Class Submarines (1989-2004)". Danish Naval History. Retrieved 6 July 2010.
- "Nyt om undervandsbåden Nautilus ("News about the submarine Nautilus")". Ingeniøren (in Danish). Retrieved 6 July 2010.
- Jensen, Mette Buck. Reaching Nexø Ing.dk (Danish), 1 September 2010. Retrieved: 1 September 2010.
-  Ing.dk (Danish), 20 January 2011.