1. What are Cubemaps?
2. Building Cubemaps

Cubemaps is a new concept to Source mapping. Due to the extensive use of models in Source maps and the support of specularity in the Source rendering engine, cubemaps are essential in ensuring that the models and any other reflective texture appear correctly. Without cubemaps, the models in a dark room would appear "lit up" and glow in the dark. In addtion, reflective surfaces would have a bluish/whitish sheen to them.

This is readily noticeable if you spawn as a CT and use your default gun model (the USP) to reflect the specularity rendering of the map. The reflective surface of the tiled floor texture also looks dramatically different with and without cubemaps:

Without cubemaps

With cubemaps (after buildcubemaps)

The bottom line is that your map would look pretty ugly without building proper cubemaps. So we'll load up our firstmap and add some cubemaps to it.