The Disk that forms the body of Worldstree is some 8,000 miles across, but only 20 miles thick. In the middle layer of this, the Roots of the Worldstree form a thick net that reaches all the way to the Ringwall Mountains. The Roots maintain the same Gravitic Principles of the Tree, and are responsible for holding the Disk in it's proper shape.

Worldstree in no way follows Rational Geology. It can be generalized that the Middle Layer that surrounds the Roots is mostly bedrock and metallic deposits (especially Adamantium), and that the Upper Level is mostly earth and water, but there are pockets of soil scattered throughout the Middle Layer, even some on the Roots themselves. There are also innumerable caves and open areas randomly scattered through the Upper and Middle Layers. The Lower Layer is unvisited.

The Rootsworld itself is the system of caves, carverns and pockets that dot the Top Layers. Many of these areas were created by the actions of water and/or Rockwyrms, but many others have no apparent origin. Rootsworld is unlike terrestrial caves in everyway. Firstly, it is rarely dark, due to Ubiquitous Luminescent Fungi. These same Ulfs act to refresh the air and can even be eaten, meaning that the Underneath is ripe to support life. Rootsworld has also been inhabited by Sentient Races for eons, and there are few areas that are inaccessible. The temperature does not rise as one goes deeper, but the Attractive Force does becomes more erratic. Standing on a Deeproot, one will weigh 1.5 times as much, but the Force extends only a few yards, and it is possible to fall right through the Midlevel. Thus the caverns in the Middle Layers can be odd indeed. Although there are stalactites and stalagmites, there are far more exotic formations, too. There are also many underground rivers and lakes, and water tends to flow downwards counterclockwise.

As with the rest of Worldstree, the vast majority of creatures in the Underneath are Sentient. Foremost are the Damethear (Dark Elves), but there are also Diggers, Celenes, Chirps, Polypedes, Dragons, and a bizarre form of Dryad associated with the giant Fungi that make up the forests of Underneath. Most of these creatures are Evil and of the Iron Alliance, but that does not prevent continuous strife and predation.