A small entrance to a large network of caves is all that the untrained eye would see if they stumbled onto the dark opening to an underground world. The nearby Menos forest is a deterrent to the average passerby, and in fact the tunnels and caves do connect in a fair few places to those massive caverns filled with the ever hungry Menos.
Those brave, or foolhardy, enough to venture into the caves would be greeting by cold, damp, and frankly unpleasant conditions as they moved deeper. Raxor edged rock formations growing from the floor to the ceiling, threatening to tear the unwary to shreds with a poorly placed step.
Deeper into the cave the tunnels would separate, moving left and right, higher and deeper into the darkness until deep within the cave there would be a small light. Those lights, spaced evenly along the wall of the cave were small generators harnessing the reishi directly from the air to light the tunnels as they crawl deeper into the sand of Hueco Mundo. These lights are placed at precise intervals despite the turning and changing of the various tunnels leading those that don't know the way deep into the nest of traps and pitfalls that are set in the various passageways.
After braving the traps and pathways those lucky few would set eyes on a massive steel door that opens to the laboratory and workshop of Hueco Mundo's resident tinkerer. They would be met first with his companion, Daymen, an injured hollow that has traveled with Tacitus from the first day that he awakened in this realm. The individual caverns within the lab are filled with panels and computers, beeping and processing any information fed in by the network of sensors nestled in the cave and built over the last months by the scientist. Projects, designs and half baked inventions cover many of the counters and workbenches around the lab, leading to a feeling of organized chaos.
Attached to the main lab/workshop is a larger cavern that is filled with raw materials, brought from the corners of Hueco Mundo by helpful hollow servants of an ally, alongside a massive military grade 3D printer. This device was the source of the original boost in technology that Tacitus had been able to gain, and the starting point of the small technological revolution taking place within the lab and caves attached.
A third cave, smaller and isolated from the others to keep it cool and dry, contains all the computers necessary to plug away at Tacitus' various theories and designs as well as monitoring the sensor devices created and installed by the scientist over the course of his inventing.
-Anyone with questions is free to ask. Please PM either "aMalfunction" or "TacitusSegador" on reddit, or hit me up in Skype chat.