Luminescent Biomass

The Luminescent Biomass is a large glowing orb located in the ocean cavern that lies beneath the city of Rapture. The light eminating from the biomass is reminiscent of the Northenrn lights and can clearly be seen surrounding the Fontaine Futuristics building and even the city itself. The radiation from the biomass is what originally mutated the native sea slug and allowed them to produce ADAM.

Bioshock 2
The Luminescent Biomass can be seen by Subject Delta when looking out of the windows from Persephone, the radiating light is so strong that it iluminates the entire cavern. When the Rapture Family detonate bombs around the detention center under orders from Dr. Sofia Lamb, the entire facility is sent sinking down into the glowing orb below it.

Behind the Scenes

 * Although the Luminescent Biomass is never explained in any of the games in the Bioshock series, it's purpose was revealed by one of the game's developers, Steve Gaynor.