For nearly a hundred years, people have built hospital ships, the size of a city block, on the ocean. And these floating infirmaries have saved countless lives and helped heal travelers on the ocean.

The U.S.S. Red Rover, the first U.S. Naval Hospital, converted from a Confederate paddle steamer

(via U.S. National Library of Medicine and Harper's Weekly, May 9, 1863)

A hospital attached to a floating battery (an ironclad vessel) built to attack Fort Sumter from the water, designed by Dr. Columbus DaVega during the Civil War in Charleston, South Carolina

(via The Medical College of the State of South Carolina)

HMS Atlas, a 91-gun second rate ship which was never completed. It was converted into a hospital ship and used by the Metropolitan Asylums Board between 1881 and 1904.

(via Dartford Hospital Histories and Wikimedia Commons)

PS Castalia, originally a twin-hulled paddle steamer built in 1874, converted to a hospital ship in 1883, and used by the Metropolitan Asylums Board until 1905.

(via Dartford Hospital Histories and Anarchadia)

A pontoon hospital, off Jarrow on the River Tyne, used between 1886 and 1930

(via Skyscrapercity)

The Steam Yacht Erin, used as a hospital ship during WWI

(Photo by Hulton Archive/Getty Images)

A Red Cross hospital ship named New York in Falmouth Harbour, UK, c. 1915

(Photo by Hulton Archive/Getty Images)

RMS Britannic, the largest Olympic-class ocean liner of White Star Line, the sister ship of RMS Titanic and RMS Olympic, launched in February 1914, used as a hospital ship between 23 December 1915 and 21 November 1916, when it sank after an explosion caused by an underwater mine.

(via Wikimedia Commons)

The sixth USS Relief with a bed capacity of 550, commissioned in December 1920, sold for scrap in March 1948

(via Navsource)

A nurse tends to young patients in a river ambulance for sufferers from infectious diseases such as tuberculosis, in July 1922. The ambulance belongs to the London Metropolitan Asylum Board.

(Photo by Topical Press Agency/Getty Images)

The Lloyd l Seaman, an engineless vessel used as a floating hospital for sick children, and a cruise ship for elderly citizens under Department of Welfare sponsorship, c. 1950.

(Photo by Al Barry/Three Lions/Getty Images)

SS Hope, originally built as a US Navy hospital ship, USS Consolation in 1945. It was decommissioned in 1955, but served again between 1960 and 1974, when it was operated by Project HOPE.

The SS Hope had its own freshwater plant named the Iron Cow, a machine that combined distilled seawater with butterfat and milk solids, producing 1,000 gallons (3,785 liters) of milk a day.

(via Naval Historical Center, Navsource)

The 894-feet (272 m) USNS Comfort and USNS Mercy, the largest hospital ships ever. Both of them have 12 operating rooms and 1,000 beds, and were built as oil tankers in 1976.

(Photos by Gabriel R. Piper/U.S. Navy/Getty Images, Jim Watson/Getty Images and Steve Helber/AP)

MV Africa Mercy, the world's largest (498 ft 8 in or 152 m long) non-governmental floating hospital, converted from a former Danish rail ferry named Dronning Ingrid (Queen Ingrid), in service since 2007. The eight-deck MV Africa Mercy has six operating rooms, a recovery ward with 78 beds, two CT scanners, an ophthalmic unit, an ICU and some laboratories.

There are 126 cabins for 484 crew members including families, a library, a small supermarket, a Starbucks cafe, a restaurant and a school for all ages.

(via Wikimedia Commons)

The Type 920 Class (Peace Ark or Daishandao) of the People's Liberation Army, China, built by Guangzhou Shipyard International Company Limited, launched in 2007, commissioned in December 2008.

Only one ship was built, which has 300 beds, 20 ICUs and 8 operating rooms.

(via Wikimedia Commons)