Hollow Earth

The Hollow Earth, sometimes called the Inner Umbra, is a Realm or set of Realms occupying the inside of the planet Earth. It is speculated to have some connection to the Shade Realm of Matter. Gravity and other physical laws work differently within Hollow Earth, and it is still under active exploration by both the Sons of Ether and the Void Engineers.

Overview
Reaching Hollow Earth is easier than most any other Umbral realm; while there is a barrier similar to the Gauntlet in place, it is far weaker, and even Sleepers may accidentally breach it. Numerous physical portals exist, ranging from underground tunnels to undersea volcanoes to a certain New York City elevator. The Technocracy has tried its best to seal these entrances, as the Hollow Earth does not conform to the Consensus.

The internal geography of Hollow Earth is difficult to explain; the regions listed below may be their own Realms, or may be overlapping parts of a single, gloriously diverse Realm. An inner sun lights the sky most of the time, though from some vantage points it rises and sets (somehow) while from others it remains static. Gravity also behaves unpredictably.

Biotopia
This lush realm is home to a wide variety of extinct plants, animals and human cultures.

The Dero Lairs
Deros are a genetically engineered "robot" species (in the sense of "worker" rather than "mechanical construct") who dwell in underground caves. They were created by the Sons of Ether before their defection to the Traditions, and still possess and use Etherite technology. They also have a fanatical hatred of surface dwellers and regularly conspire to cause as much death and destruction as possible. A much smaller number of the robot species, called teros, are friendly to humans and attempt to undermine the deros when they are able.

The Rainbow City
A floating city of crystal inhabited by scholar-mages called the Hidden Masters, this city is only accessible by climbing a narrow, twisting stairway made of nearly transparent crystal. It was either an ally or a colony of Agharta.

Agharta
This Realm was once a paradise inhabited by the Goro Monks, and provided the inspiration for the adventure tales of Doc Eon and his Terrific Trio. After World War Two, however, Nazis invaded Agharta and the region has been rocked by warfare ever since.