The Underways

The Underways are a series of artificially reinforced tunnels that are regularly unearthed by mining operations throughout the Iron Hills. These tunnels are all remarkably large and straight, and feature elaborately carved stone support columns that resist any blow from a pickaxe. The walls, ceiling and floor are generally either very smooth stone or engraved with murals or other decorations.
Nobody knows where these tunnels lead: explorers sent down the tunnels that return report nothing but endless straight corridor. Some report a strange mental ...pressure... that caused them to turn back, but these descriptions don't match anything that anyone who's studied the matter can identify. Most don't come back at all
For centuries, most of these tunnels have been blocked up upon discovery, mines collapsing their own access tunnel that breached the Underway, for fear of what lurks within - whatever it is that causes so many expeditions to disappear. Scholars theorize that these tunnels are very old, but they have no idea how true that is.

Purpose

The Underways are, in fact, highways - major roads connecting the cities of a vast subterranian network of cultures. Similar paths could be found elsewhere, but scattered below the Iron Hills is a complex network of cities and outposts - a major trading hub for this ancient network. These cities predate even the First Empire, protected from the threat of magical storms not by anchors, but by powerful ritual magic wrought by their Archmagi. These rituals provided protection for chambers being used as cities, and along the artificial highways linking them, shunting the effects of these storms out into other chambers, and also formed the backbone of defense for these cities.
That magic stopped working when the Anchors were developed and put in place in the world above. By now, all of those cities have long since fallen - the anchors suppress magical storms just fine, but they do nothing to stem the tide of nightmarish predators and worse that made a home of their own in these caves.

Geography

The Underways themselves are long and straight tunnels, still standing centuries after their construction, but completely barren of life or useful navigational aids. Scholars that examine the tunnels all agree that they were crafted magically, with normal tool marks only visible in the carvings found in tunnels and on the regularly positioned support pillars. These pillars do prove to be confusing to surface dwellers, however: how can one chisel in such fine carvings as if it were marble, when the pillar as a whole resists any attempt to break it. The best guess is some magical effect, but unlike carving the tunnels in the first place, nobody knows how ordinary stone could be enhanced to such a degree.
The cities they connect to are more interesting, and far more dangerous. Many buildings are structurally intact, made of the same nearly indescructable stone as the Underway pillars, those that aren't are largely collapsed and ruined. Sections of these caverns are dominated by narrow passages between high walled buildings covered with carved decorations, interspersed with partially or completely collapsed ruins, others are swathes of open ground covered with fungi or underground rivers or lakes.
Lastly, there are the wild caves - winding passages and caverns where the earth has cracked open and nobody has visited in countless millennia. Here the terrain is governed by heat. Warm, wet caverns support colonies of mould, which in turn support forests of fungi and a vibrant ecosystem. Cool or dry caverns are more sparsely populated sometimes featuring a few fungi or are just barren stone. Regardless, footing in these wild caves is usually trecherous at best, and all but impassable at worst.

Ecosystem

Caverns that are both warm and wet are the foundation of life in the Underways. The warmth and moisture are enough to sustain several species of slime mould that have propagated throughout the caverns. This is then consumed by an array of small herbivores, who in turn support an array of predators. Organic matter from all these sources provides the necessary components for a wide array of fungi that prove to be every bit as complex as plant life on the surface.
In wild caves, creatures that feed on the mould and the fungi creep about stealthily to feed before slinking away. Some hide their presence by selecting rooms with ambient temperature similar to their body temperature, making them difficult to detect for predators who hunt heat sources. Others have tough digging claws that make it a simple matter for them to burrow into the rock, packing in the holes behind them so that they are not disturbed. Others simply stay on the move, sleeping in short fits as if their day and night were an hour or less long.
In the cities, the only source of food is out in the open - the wide open areas where the residents once raised tamed creatures as a food supply. Now these places are overgrown, but with the terrain unbroken it is a perfect hunting ground for predators. Grazing animals must sneak out from the buildings they hide in to sleep and be extremely stealthy or very quick. The most successful of these are those that can gather food for later, filling its arms with food and returning to its den.
Carnivores abound in this environment. Feeding by ambush, by stalking, or by picking and pursuing a particular target unto exhaustion, all of these means of predation come into play. Some of these are just natural beasts that arose in these caverns, but many are extraplanar monsters, descended from beings brought here by magical storms. But since the imposition of the Anchors, the ecosystem has settled into an uneasy equilibrium - prey knowing how to respond to threats new and old, and with nothing new arriving at random intervals to disrupt the balance.