Glorantha is a magical world, not a planet floating in space. Its universe obeys other laws than those of terrestrial physics. The Inner World, the world of mortals, is shaped like a huge cube of earth floating in a limitless sea of water. Two major continents, Genertela and Pamaltela, and a myriad of minor islands rise above the surface of the infinite sea.
The Sky World lies beyond the dome of the sky. At night, the stars show where the magical denizens of the Sky World peep through the Sky Dome to guard their followers below. Similarly, the Underworld is populated by immortal creatures, as well as by evil dreams and by the souls of the dead and unborn.
The Sun rises from the east each morning, then passes over the air, and across the sky to set each evening in the west. It then travels beneath the seas and the earth through the Underworld, and finally again up to the eastern lands of the dawn.
The world is populated by magical entities who personify fundamental powers and cosmic truths, spirits of diverse origin, power and benevolence, and feared manifestations of Chaos (foul-seeming and otherwise). Many of these beings are alive in every sense, but many are not. Most people live only in the mundane world, content to let priests, shamans, and wizards protect them. But the influence of the magical world is always there.
from Guide to Glorantha, Volume 1, by Greg Stafford, Jeff Richards and Sandy Petersen