The mountain pass is west of Lurleen and east of the Open Prairie. There are two entrances to the mine here. The Abandoned Complex is here too, but you'll need some sort of authority to get in, not to mention a map to it. If you go exploring you might even find a cool place to have a blast.
Bandits: most of which will throw cherry bombs about. These can catch you unawares and do a surprising amount of damage so move quickly and carefully.
Yetis: Nasty bastards, beat protection is recommended.
Bunnies: Now why would you want to kill a helpless little bunny rabbit? These are about as tough as you would expect.
Both the bloodhound and yeti skin mutations can be found here. There is also a container that spawns two (usually) decent crafting parts.
A detailed map with descriptions can be found here.