Caves

On the sides of many of the mountains, there are hundreds of caves. Many burrow deep into the mountains and even connect. They are dangerous and most have not been searched, though the ones that have been passed through are said to have beautiful gems unlike any others found, and strange pokemon who's isolation in the cave has adapted them to the darkness. Its easiest to pass through one certain cave(unfortunately one of the least inhabited caves) in order to get across the mountains. Perhaps you can sneak into one of the prettier caves if you're really that curious?