A scientific discovery upset the balance of the society. Two scientists studying ocean-floor dwelling creatures discovered a species of
sea slug that secretes pure
stem cells. These could be used to enhance one's body, improving physical or mental capabilities, curing diseases and healing injuries. A young entrepreneur named Fontaine invested early on in the research to gain control over the material. The substance, dubbed "Adam", became so sought after in the society, that it became the dominant currency of the city. A "full-scale genetic arms race" broke out between Ryan and Fontaine as Fontaine's monopoly on Adam threatened the current social structure. Ryan eventually won, but everyone in the city was permanently changed. During the war, it was discovered that Adam could be used to modify one's body, combining technology and mutations to adapt and survive the conflict, but losing their humanity in the process. During the conflict, all natural sources of Adam were destroyed, which eventually resulted in a major shift in the "ecology" of the city, as all inhabitants had become biologically dependent on Adam to survive.
When the player arrives in Rapture, the city is in a state of disrepair, its vast underground living areas and laboratory complexes scarred by the effects of the civil war and poisoned by biological weapons. Low-level
flooding is also found to be a problem, as the use of high-powered
explosives and
corrosion weapons during the war created small breaches in the city's sea walls, allowing the sea to enter some of the buildings.
As the player descends through the underwater city, he or she will explore the many levels of the giant undersea base, including the huge living quarters modules, the multi-floored scientific headquarters and the undersea
monorail transport network. Remnants of the last days remain in notes and recordings made by the citizens before and during the collapse of Rapture's society. Not only does this provide background, it also opens new avenues in the player's interaction with the ecology of Rapture.