The Inventium is a Realm of the High Umbra full of every invention humankind has ever dreamed of. These inventions manifest with an audible popping sound and drop gently into massive piles that fill the Realm in all directions.
In modern nights, at least, access to the Inventium involves locating a nondescript metal foot locker floating in the High Umbra. There is a warning inscribed on the lid warning off travelers. Those who disregard the warning and climb into the locker find themselves descending a staircase into the Inventium.
The Inventium was first discovered in 424 BCE by a group of mages called the Sons of Thucydides. They recognized what the Realm represented, and though none of the actual inventions could be brought back across the Gauntlet, schematics and notes could be. The descendants of the Sons of Thucydides are now called the Scavengers, and they continue to make a tidy profit selling their second-hand inventions; some were born in the Inventium and may not ever have visited Earth.
A friend of the Sons, a mage named Herodotus, joined them there with a different purpose. Along with several colleagues, Herodotus hoped to document the contents of the Inventium, not for profit but to advance human knowledge. These mages became known as the Historians, and their leaders continued to adopt the name Herodotus as a title. After the formation of the Council of Nine Mystic Traditions, the Historians welcomed members from the four Traditions who shared their interest in the Inventium (Verbena, the Cult of Ecstasy, the Order of Hermes and the Celestial Chorus). Life in the Inventium is not a prospect most mages relish, however, and thus vacancies in the Historians go unfilled for years at a time, or are filled with disgruntled exiles rather than eager volunteers.
The Technocratic Union discovered the Inventium in the 19th century, and in 1895 they built the Hackworth Technological Compilation Facility near the center of the Realm. They have attempted to drive out the Historians and the Scavengers by force, without success. Rather than cataloguing or exploiting the technology of the Inventium, the Technocrats identify inventions that violate their goals for the Consensus and use them to track down threat to the Time Table for elimination.
The Sons of Ether also maintain a presence in the Inventium, which is devoted in equal parts to uncovering ancient inventions, and harassing the Technocrats at Hackworth. Their local headquarters is a Gothic-looking mansion called Exington Hall.