Andrew Ryan began the construction of Rapture in 1946. Using his own private steam-liner, the Olympian, he transported building supplies to the coordinates where the city would be built, roughly located at 63° 2' N, 29° 55' W in the North Atlantic ocean. A building platform, nicknamed "the Sinker", was lowered from the Olympian to bring the supplies and crew to the site. Science would thrive undisturbed by intervention from governments, religious institutions or other social agencies. Rapture was designed to be entirely self-supporting, with all of its electricity, food production, water purification and defense systems powered by undersea volcanic vents. People from around the world emigrated to the hidden city during the years between 1946 and 1952, sparking conspiracy theories on the surface about mass disappearances that came to be known as "The Vanishing". Rapture's population was composed of the people Ryan viewed as the "best examples" of humankind.
Unfortunately, the city plans did not include accommodation for the large number of workers who had labored to construct it. While the artistic and scientific elite from around the world emigrated to fill the metropolis, said workers were left in leaking, crowded, and what was meant to be "temporary" housing in Pauper's Drop. Ryan and his Central Council took little notice of the stark contrast between the poor and the intellectual upper crust, mostly ignoring it. Ryan did, however, listen to advice that the citizens were becoming anxious from the isolation of the city and the lack of sunlight. Thus, Sofia Lamb, a renowned psychiatrist on the surface, was hired by Ryan Industries at Andrew Ryan's personal request, and invited down to Rapture to help individuals cope with life under the sea. Lamb's utilitarian beliefs largely clashed with the free market ideal of Rapture, and she devoted much of her efforts to the poorer citizens, in this way, becoming a significant political rival of Ryan.
During that same time period, between 1949 and 1951, Dr. Brigid Tenenbaum happened to be passing through the docks in Neptune's Bounty when she noticed an individual, whose hands she knew had been paralyzed in the war, was playing catch. After inquiring as to the reason behind the revival of his hands, she was told he had been bitten by a sea slug. Tenenbaum asked the man for the slug, knowing that she had discovered something amazing, and began to search for a benefactor who would fund her research.
Scoffed at by all other scientists in Rapture, Tenenbaum approached Frank Fontaine, a businessman with a distinguished name in Neptune's Bounty. He agreed to fund Tenenbaum's research and development, as long as he was given full rights to profit from the findings. He funded her research using the fortune he had acquired from his smuggling ring, bringing contraband items down to Rapture from the surface. Tenenbaum's research quickly began to take off. She discovered that the substance the sea slugs secreted behaved like stem cells, and that by using it, she could manipulate DNA entirely. She was able to "bend the double-helix", cure diseases, and give organisms amazing powers. The substance was thought to be a "rebirth" for humanity, and it was dubbed "ADAM".
Shortly after the discovery of ADAM, the scientists made another breakthrough: If the slug was implanted into a host, it would produce twenty to thirty times the original amount of ADAM. Presumably, they tried to insert the slug into the stomachs of various other test subjects, but only one type of subject seemed to work well with the slug: young girls. After this discovery, they started to implant the slugs into little girls, but they needed a ready supply of hosts if they were going to mass-produce ADAM as Fontaine wanted. Taking inspiration from Sofia Lamb's earlier charity work, Fontaine set up the "Little Sister's Orphanage", advertising it as a place where financially pressed families could send their little girls for care and schooling. The transformed girls eventually came to be known by the orphanage name as Little Sisters.
