Mount Olympus towers up from the center of the earth. Here the major gods live and hold court. The myths are somewhat vague on weather it is an actual mountain or a region of the heavens.
The underworld is hidden in the earth. It is the kingdom of the dead and ruled over by Hades. Hades is a greedy god who is greatly concerned with increasing his subjects. Those whose calling increase the number of dead are seen favorably. The Erinnyes are welcomed guests. He is exceedingly disinclined to allow any of his subjects leave.
For most, life in the underworld is not particularly unpleasent. It is rather like a miserable dream, full of shadows, without sunlight or hope. A joyless place where the dead slowly fade into nothingness.
Tartarus lies far beneth the disk of the world. Deeper then Hades kingdom of the underworld. It is used as the ultimate of prisons, unpleasent and inaccessable.