So I'm working on a home-brew D&D setting and I have this idea for the my campaign to take place in a large isolated Valley completely surrounded by mountains. I also had the idea to have these mountains be at the edge of the known world in a cold otherwise inhospitable environment. The valley in question however would be temperate which would spark an initiative to settle the region, causing many brave individuals to make a hard trek for weeks to reach said location motivated by the promise of a land filled with many natural resources.
I am currently trying to come up with a geological explanation as to why the valley would be temperate. I think it would also be cool for said explanation to be a mystery that the players have the opportunity to uncover. Any thoughts would be appreciated. (Also, my world is Loosely Forgotten Realms based)