The country that occupies the Valley, Pocalo, is divided into provinces, and each province is stacked on top of the other. The massive structure that houses all these provinces was built up slowly over the course of thousands of years. For most of that time, the only way to get from one province to another was to take a long trek up or down the mountains that anchor the structure.
The safest way to go was via mountain tunnels. These tunnels were dug and maintained by the Daraz, who build their cities inside the mountains. They leased the tunnels to the humans in exchange for various goods and services.
A scant couple hundred years ago, humans perfected the technology to move elevators between the provinces. The early elevators could only connect two provinces; later technological advances brought elevators that could connect 3 or 4 provinces. As elevator technology improved, the need for the tunnels waned in favor of the much faster vertical transportation.
The Daraz had little incentive to maintain the tunnels with no one using them much, and shifted to other forms of trade with the humans.
Getting on an elevator typically requires going through a checkpoint, though, and may require an official passport for boarding. What to do if you are perhaps interested in avoiding any sort of official scrutiny in the course of your travels? Like, say, maybe you deal in goods you might have obtained under less than legal circumstances?
Well, it’s certainly slower going, but here are all these old tunnels no one uses anymore, cough, ahem.
Yes, there have been many attempts to seal off the tunnels over the years, but somehow or other they always get unsealed, and the resources just aren’t there to guard them 24/7. Hence the tunnels tend to be left alone much of the time, should someone wish to slip from one province to the next while attracting minimal attention.