The Grackle said:Maybe it's just really cold everywhere except in the valley. If there was less atmosphere than Earth, a 4 mile deep canyon might be the only hospitable place. I think there was a Larry Niven novel w/a planet like that.
The classic example is The Night Land by William Hope Hodgson ca 1910, where the sun has died but the ca 100 mile deep rift valley is still habitably warm...
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)