Tutrabel Bridge

It's hard to tell that the Big Charry region was the largest seat of those underground.  Hard from ground level, hard from the air.  In the whole region, there is a single structure that has any apparent features aboveground that was used solely for those underground.  Tutrabel mountain has a bridge that comes out of its face, about halfway up to its peak (the peak is  6922 feet, being the third highest mountain in the region, Big Charry is the highest at 9138 feet) connecting to a tall elevator tower that forms the center of Tutrabel City.  This structure was made by human hands, but to the specifications of those underground; it's completely unadorned, plain, almost crude looking.  At the same time, it is so distinct from human engineering that it seems strange that a race which had done everything in their power to hide all traces of themselves away, even (especially) from their slaves, would want this.

Tutrable was not even that important a city to those underground, as far as anyone could tell.  It was small, had a comparatively paltry livestock yield and very little in the way of mineral wealth.  It's possible the bridge and the tower were set up as a decoy for some presumed enemy (who or what is still nothing we know).  The elevators in the tower, drawn by water power, still function, and the doorway into the mountain remains open, but no one has been able to get inside.  A cold wind issues from the doorway, which, at its source is colder and faster than any other conditions found in this world, and, at present, no one has had the time or resources in place to create a method to defeat those winds (though there are designs, some of which seem promising).

Explorers hoping to learn the secrets of those underground and how they met their end have flocked to Tutrabel City, but no one has been able to establish a foothold strong enough to even begin exploring the tower and city, let alone dare the wind.  Competition between expeditions seems to be the most common cause of expedition failure, but it seems clear that there are other agencies operating in the city, about which no good accounts have come.