Mountain base was a test map used in the update video "[Unity 3D] EliteSquad Project Updates". The map was created by NSDesignGames. You can view the video showing off the map and some of the updates in the video here.
The map (to date: 10/20/11) is so far the biggest one released. The map features wide open areas with very little cover. Also, there are sniper posts around the map that the player can access as shown in the video.