The planet of Torus is a very flat disk, rim-loaded to cause a gravitational force towards the outside edge, with a large hole punched through the center. The atmosphere (presumably) extends in all directions, and stars and a sun are visible from opposite sides of the disk. Torus has a geosynchronus orbit so that one side is always facing the sun. The side on which the sun shines is called the Light Side, and the other (from which the stars are visible) is the Dark Side.
The Dark Side of the Torus Age is the more remarkable of the two. Its most prominent feature is a large column of water which emanates from the hole in the center of the disk and streams into the night sky. Eventually, it evaporates and is gravitationally attracted back to the disk. The pillar of water originates from a dream of Catherine's to "...swim among the stars." Other notable features of the Dark Side include the link-in point and a wide variety of unusual unnamed organisms, including a rather cat-like flower, mitotic snakes, and exotically-crested birds.
The Light Side of Torus is most notable for the large lake, covering most of the area and surrounded by mountains, which drains through a large whirlpool to feed the jet of water on the other side. The mouth of this whirlpool is ringed by several rocky islands, covered in flowers. Among these flowers are the "delicate blue flowers" known as ephemerals also growing in the Cleft. This was probably Anna's idea.
The explanation for the strange behavior of water in this Age is described by Atrus as an interplay of two forces, a 'jet force' of unknown origin that forces the water through the hole on the Light Side and away from the Torus in a constant stream until it evaporates, and the gravitational force which attracts the water vapor back until it condenses around the mountainous rim of the Light Side and rains back into the lake.
"Torus" is an unofficial name. What Catherine called the Age is unknown.
- Myst: The Book of Atrus