Occlusion Culling (Pro Version Only)
Occlusion Culling is a method available in many game engines (including Unity) that can help save system resources for large maps.
Occlusion Culling means that the camera will only render what is visible to the camera. Any object that is not viewable to the camera will not be rendered.
This is done by first breaking the level down into zones, with each zone having large objects (like walls) that block vision to other zones. Then, after some more set-up, Unity determines which zones are visible to the camera and sets them to render. Any zones that cannot be seen will not be rendered until the camera can see them.
The Occlusion Culling menu is at Tools -> Occlusion Culling.
Learn more about Occlusion Culling as well as how to implement it here: http://unity3d.com/support/documentation/Manual/Occlusion%20Culling.html