Amalgam

The Amalgam is very poorly understood. It's a huge, impenetrable tangle of techmaturgical machinery deep below Tellura’s earth and stone, serving both as a barrier blocking downwards progress, and a network that links all the ACEs into one enormous engine. It was presumably designed by the Architects. A ways beneath it’s nearly impenetrable surface layer there are huge caverns of empty, unnatural darkness, many of which are of immeasurable depth. It is known that the further one travels in the Amalgam, the less reliably time flows, and the longer one spends inside, spaces tend to shift and distort. Back when explorers were less knowledgeable and researchers more tenacious, parties would frequently go missing. Most of the time no trace would be found, these cases were the least disturbing. Occasionally missing members would be found partially inserted into solid walls, sometimes still alive. A group might discover bones in a sealed room dated to be hundreds of thousands of years old, only to learn via dental or fracture records, that they belonged to a missing colleague.

There are huge pockets of machines that seem older, or at least much less sophisticated than the rest of the Amalgam. The twisting, rusty metal sprawls like a coral forest through the depths, pumping ocean's worth of molten material on unknowable journeys. These dilapidated, rodent infested engines are believed to have been left behind by whatever forces protect and contort the rest of the Amalgam leaving them relatively stable. These regions, referred to as the Nether, are frequently mined and salvaged for the abundance of refined metals they possess, as well as rare crystalline structures useful as power sources.