The Gap

The Gap is a strange name for the feature is describes. Much of the rock under the rich topsoil and drit of the southern Plains of Kataru is permeable and easily eroded. As a consequence, there are caves littering the landscape. Some of them are small, with barely enough room for a single ravage bear to hunker down, while others open into vast systems with miles of tunnels and rooms.
The Gap is such a system. No comprehensive attempt has been made to map it, but estimates range from 250 to 500 miles of tunnels, and it's clear that not all of them are natural formations in the rock. Throughout these tunnels are dozens prior world ruins, and the people of Phrygia-by-the-Gap have barely scratched the surface of them. Literally - the synth miners have spent decades just tearing down walls and selling them as building components, leaving the interior of the ruins (mostly) untouched.
Sometimes an intrepid and hungry miner will decide to go a little deeper, and this has alternately paid off well and cost a lot. Artifacts and cyphers about in the ruins, but so too do defensive systems, seemingly-sentient fungus, and energy fields that buzz with warning when approached too closely. The Aeon Priests of the Phrygian Sound are known to pay well for artifacts recovered from The Gap, but very few people are willing to do more than mine the synth along walls and roadways. The potential wealth of the prior worlds is there for the taking.