HMHS Britannic (1914), the third and largest Olympic-class ocean liner of the White Star Line, sister ship of RMS Olympic and RMS Titanic, sank in 1916 after hitting a mine with the loss of 30 lives. Although the White Star Line always denied what it called a 'legend', most sources say that the ship was originally intended to be named Gigantic (this is supported by a White Star pamphlet that advertised 'The Gigantic'). In the aftermath of the Titanic disaster, the name was changed to Britannic, making it the second of three ships to be so named.

Britannic was launched on 26 February 1914 at the Harland and Wolff shipyard in Belfast and fitting out began. In August 1914, before Britannic could commence trans atlantic service between New York and Southampton, World War I began. Immediately, all shipyards with Admiralty contracts were given top priority to use available raw materials. All civil contracts (including the Britannic) were slowed down.

The need for increased tonnage grew critical as military operations extended to the eastern Mediterranean. In May 1915, Britannic completed mooring trials of her engines, and was prepared for emergency entrance into service with as little as four weeks notice. The same month also saw the first major loss of a civilian ocean vessel when the Cunard liner RMS Lusitania was torpedoed near the Irish coast by a German submarine.
The following month, the British Admiralty decided to use recently requisitioned passenger liners as troop transports during the Gallipoli campaign (also called the Dardanelles service). The first to sail were Cunard's RMS Mauretania and RMS Aquitania.

As the Gallipoli landings proved to be disastrous and the casualties mounted, the need for large hospital ships for treatment and evacuation of wounded became evident. RMS Aquitania was diverted to hospital ship duties in August (her place as a troop transport would be taken by the RMS Olympic in September) and on 13 November 1915, Britannic was requisitioned as a hospital ship from her storage location at Belfast. Repainted white with large red crosses and a horizontal green stripe, she was renamed HMHS (His Majesty's Hospital Ship) Britannic and placed under the command of Captain Charles A. Bartlett (1868 - 1945).

After completing five successful voyages to the Middle Eastern theatre and back to the United Kingdom transporting the sick and wounded, Britannic departed Southampton for Lemnos at 14:23 on 12 November 1916, her sixth voyage to the Mediterranean Sea. The Britannic passed Gibraltar around midnight on 15 November and arrived at Naples on the morning of 17 November for her usual coaling and water refuelling stop, completing the first stage of her mission.
A storm kept the ship at Naples until Sunday afternoon, when Captain Bartlett decided to take advantage of a brief break in the weather and continue on. The seas rose once again just as Britannic left the port but by next morning the storms died and the ship passed the Strait of Messina without problems. Cape Matapan was rounded during the first hours of Tuesday, 21 November. By the morning Britannic was steaming at full speed (around 21 knots) into the Kea Channel, between Cape Sounion (the southernmost point of Attica, the prefecture that includes Athens) and the island of Kea. At 08:12 on Tuesday, 21 November 1916, a loud explosion shook the ship. The reaction in the dining room was immediate; doctors and nurses left instantly for their posts. Captain Bartlett and Chief Officer Hume were on the bridge at the time, and the gravity of the situation was soon evident. The first reports were frightening. The explosion had taken place on the starboard side between holds two and three, but the force of the explosion had damaged the watertight bulkhead between hold one and the forepeak. That meant that the first four watertight compartments were filling rapidly with water. To make things worse, the firemen's tunnel connecting the firemen's quarters in the bow with boiler room six had also been seriously damaged and water was flowing into that boiler room.Bartlett ordered the watertight doors closed, sent a distress signal and ordered the crew to prepare the lifeboats. Unfortunately, another surprise was waiting. Along with the damaged watertight door of the firemen's tunnel, the watertight door between boiler rooms six and five also failed to close properly for an unknown reason. Now water was flowing further aft into boiler room five. The Britannic had reached her flooding limit. She could stay afloat (motionless) with her first six watertight compartments flooded and had five watertight bulkheads rise all the way up to the B-deck.
However, there was something else that probably sealed Britannic's fate: the open portholes of the lower decks. The nurses had opened most of those portholes to ventilate the wards. As the ship's list increased, water reached this level and water began to enter aft from the bulkhead between boiler rooms five and four. With more than six compartments flooded, the Britannic could not stay afloat.

Fifteen minutes after the ship was struck the open portholes on E-deck were underwater. Water also entered the ship's aft section from the bulkhead between boiler rooms five and four. The Britannic quickly developed a serious list to starboard. To his right Bartlett saw the shores of Kea, about three miles away. He decided to make a last desperate effort to beach the ship.
At the same time, on the boat deck the crewmembers were preparing the lifeboats. Some of the boats were immediately rushed by a group of stewards and some sailors, who had started to panic. An unknown officer kept his nerve and persuaded his sailors to get out and stand by their positions near the boat stations. He decided to leave the stewards on the lifeboats as they were responsible for starting the panic and he did not want them in his way during the evacuation. However, he left one of the crew with them in order to take charge of the lifeboat after leaving the ship.

The first rescuers to arrive on the scene were the Greek fishermen from Kea on their kaikia, who picked up many men from the water. One of them, Francesco Psilas, was later paid £4 by the Admiralty for his services. At 10:00, HMS Scourge sighted the first lifeboats and ten minutes later stopped and picked up 339 survivors. HMS Heroic had arrived some minutes earlier and picked up 494. Some 150 had made it to Korissia (a community on Kea), where surviving doctors and nurses from the Britannic were trying to save the horribly mutilated men, using aprons and pieces of lifebelts to make dressings. A little barren quayside served as their operating room. Although the motor launches were quick to transport the wounded to Korissia, the first lifeboat arrived there some two hours later due to the strong current and their heavy load.

1,036 people were saved. Thirty men lost their lives in the disaster but only five were buried. The others were left in the water and their memory is honoured in memorials in Thessaloniki and London. Another twenty-four men were injured.

The wreck of HMHS Britannic is at 37°42?05?N 24°17?02?E? / ?37.70139, 24.28389 in about 400 ft (120 m) of water. It was first discovered and explored by Jacques Cousteau in 1975. The giant liner lies on her starboard side hiding the zone of impact with the mine. There is a huge hole just beneath the forward well deck.