There can be seasons even if the whole world lies beneath the surface. Because it becomes colder even under the surface. However snow is impossible, and I doubt that the inhabitants in PS celebrates cristmas, even if they do it in reality. That just wouldn\'t be serious.
If the caves lies deeply enought, then it must become warmer the longer down you travel.
The lower parts has got to be very hot and the upper ones become colder. From what I\'ve understood there will be a possibility to reach the surface, so perhaps there will be seasons after all. At least on the surface.