As its name suggests, the Middle Underdark is the central layer of the Underdark, sitting in between the Upper Underdark and the Deep Underdark. Whilst its cities and settlements are much fewer and far between than in the Upper Underdark, the Middle Underdark is still well populated, and it has a much higher density of Duergar living within its extent than the other two layers. However, it is also infamous for the much larger population of strange, predatory creatures and it is even said to have a large population of Illithids dwelling somewhere within it.
The Middle Underdark is split into four broad regions: The The Lake of Tears and its environs, the Crystal Caverns, the Burning Lakes and Still Waters. These regions are connected by miles and miles of tunnels, both monumentally large and so small that most humanoids would struggle to traverse them. On particularly noticeable difference between the Middle Underdark and its siblings above and below it, is that the air is oddly still and in most places uncomfortably frigid in this part of the Underdark. In the Upper Underdark, a constant, though often barely perceptible airflow through the tunnels is provided by its numerous openings onto the surface world, whilst the Deep Underdark's atmosphere is constantly being shifted and buffeted by the heat rising off the Lava Sea and stirring up the air currents. The atmosphere in the Middle Underdark, however is often describes as being the most claustrophobic and oppressive of all of the Underdark’s levels, with non-native travellers frequently describing how they were aware of their every breath and movement whilst traversing this alien place.
Unlike the Upper Underdark, which has a preponderance in base metals (which are still highly sought after and valuable), the Middle Underdark has the largest deposits of precious and rare metals out of all of the Underdark's three layers. Huge veins of gold, silver and platinum course through the rock of the Middle Underdark, attracting many people to try and make their fortunes in this remote, dangerous and often forlorn part of Kelbonnar’s Material Plane. The Middle Underdark does also have lucrative deposits of gemstones, though they do not occur as much here as they do in the Deep Underdark at the bottom of the Material Plane. One exception to this rule is Kinitiki, a gemstone that is only found in the Crystal Caverns located in the Middle Underdark, which is highly sought after for its beauty and its rumoured ability to provide extrasensory powers.