the Founders and the Vox Populi. These foes range from normal infantry to Heavy Hitters, more formidable enemies that act as mini-bosses throughout the game. Armed automatons scattered throughout Columbia act as a security defense system for the city. Columbia is filled with large, open spaces that offer players freedom in how to approach enemy encounters. The city is traversed on foot or via the Sky-Line, a rail system that Booker and enemies ride via special hooks. Booker can jump on, off, and between Sky-Line tracks at any time and fire one-handed weapons while riding them. The hook also serves as a melee weapon, capable of performing executions on weakened foes. Booker is aided throughout the game by Elizabeth, a computer-controlled non-player character. Elizabeth requires no protection and assists during combat by tossing helpful items to Booker as needed. She can also be directed to open Tears, bringing in items or pieces of the environment, such as medical kits or a ledge for higher ground. Elizabeth can pick locks using her hairpin to open doors or find useful items. Cash, food, medical kits, ammunition and Salts can be found scattered throughout the game's environments. Vending machines can be used to buy supplies and powerful upgrades for weapons and Vigors. Optional side-missions are also available, where the player must unlock safes or decode hidden ciphers; completing them rewards Booker with supplies and upgrades. Audio logs (Voxophones) and film projectors (Kinetoscopes) scattered through Columbia expand on the characters and events of the game. Infinite has four difficulty levels: Easy, Normal, and Hard difficulties are available to start. After beating the game on a lower difficulty level or inputting the Konami Code in the main menu, the hardest "1999 Mode" is unlocked. Enemies are much tougher, the player's navigational aid and aim assist is removed, resource management is much more crucial to survival, and death is more punishing. Development Developer Irrational Games (then 2K Boston) and publisher 2K Games released BioShock in 2007 to critical and commercial acclaim. In late 2007, 2K Games approached Irrational about a sequel. Exhausted from shipping BioShock and wary of repeating themselves by returning to the same property, staff and studio head and creative director Ken Levine were uninterested in immediately returning to another BioShock game. Instead, Irrational and 2K struck an agreement that saw select personnel from Irrational form a new studio, 2K Marin, to start work