The holodecks are 4 large holotechnology-rendering platforms, designed to create photonic facsimilies of objects and people, located on Deck 9.
When inactive, the holodeck rooms are among the largest open spaces on the ship, together with the shuttlebays and cargo bays. Holo-emitters are concealed behind a photonic-responsive grid pattern on the walls, ceiling and floor, which are used by the computer core to render the holograms in space.
When activated, the holodeck can reproduce thousands of different objects, which are stored in the computer core. Additionally, holographic programs can be given apparent 'life' by heuristic algorithms which simulate intelligence and sentience. The Challenger's computer core has been given the latest safeguards from Starfleet to reduce the chance of accidentally creating fully-sentient autonomous holograms. However, given the emergent nature of consciousness, this is an ongoing area of Federation research and ethical discussion.
Holodecks are typically used for entertainment and training purposes on board the Challenger. Equally, they can be used as emergency auxilliary sickbays.
Typically, a holodeck can accomodate up to 50 people in a static environment. However, in more complex programs, where groups of people apparently travel away from each other, the holodeck sets up duct blinds between the two groups, and renders far away objects on a 2D forcefield using autostereoscopy. In such occasions, it is recommended fewer people use the holodeck at one time, as it impacts processing speed and can lead to program crashes.