Outside of the map landscape?

[Wolfy Snackrib]

How do you create those landscapes that you can visit in minature form if you use noclip and go outside of the map? You know for example in aztec you can see all these cool kickass maps and if you go straight down under the map you see a miniature model of it, how do I create one of those?

 

[The Dark Elf]

http://www.valve-erc.com/srcsdk/Levels/3d_skyboxes.html