Tunnelers

The origin of the tunnelers is actually a mystery - nobody, themselves included, know when and where exactly did they started. The nature of the Labyrinth has managed to achieve that, splitting the tunneler early tribes into thousands of city-states, each having different myths and legends. As a result, nobody truly knows anything, and what's left is mostly false or heavily altered.
The most common theory is that the Labyrinth was populated by humans in several ways over the millenia (perhaps dating back to the times soon after the end of the Labyrinth creators, if not earlier). The first group adapted to the place somehow (perhaps with help from some genmaturgy or a supernatural entity), then it was - at some point - strengthened by a wave of surface refugees/emigrants that has at some point delved into the Labyrinth. But with group not being large enough to overwhelm the initial population, all they did was enhancing their genepool.
Then, after few centuries and another major event on the surface, the process repeated itself. Of course, there is also a possibility that an invasion on the surface (or some slave raids) has contributed to the long-term process of establishment of the tunnelers as known today.
Tunnelers have never been fully incorporated into the Grand Empire, even in its Second Era prime. As a result, the genmaturgic project that led to creation of the high humans has never reached them. They are, in an essence, baseline humans adapted to extreme environment, with significant 'structural' weaknesses when compared to the imperials living above them.

Basic Information

Anatomy

Tunnelers appear to be albino-like humans, with standard amount of limbs but white hair and pale skin. Skin occasionally appears to be slightly translucent, resulting in tunnelers with a visible sub-surface veins giving them some colouristic. The reason for this is unknown, as it cannot exactly be considered an adaptation to the environment, and in fact might be a proof for some initially supernatural cause for the tunneler development.

Genetics and Reproduction

Tunnelers genetics are notoriously unstable. They work fine on their own, however attempting to interbred with other subspecies of Mankind is notoriously difficult. It is possible, but the ratio of deformations and miscarriages is massively increased when compared to the non-crossbreeding pairs. It's slightly better with the imperials and baseline humans, but not by far.
As a result, it is speculated that the tunnelers are actually a subspecies in a transitional state between being part of Mankind and something else. They have genetically diverged (or 'were' diverged) far enough that they are a few steps (or random mutation) from becoming a branch of species on their own.
Scholars supporting the theory that most if not all other humanoid species are such far-diverged branches of Mankind typically see the tunnelers as a proof for that, although their opponents typically claim that until they lose their ability to crossbreed with other humans they aren't a proof for anything.

Additional Information

Perception and Sensory Capabilities

Contrary to what one might expect from a species living underground, tunnelers actually possess an eyesight, and it's not an awful one. They can see decently in the dark but it's not a perception method that they put a great focus on - they are very easily blinded by sudden bursts of light, and require months-long adaptation to not be constantly blinded by sunlight if they decide to leave Labyrinth.
Instead, the tunnelers seem to possess an excellent hearing, in fact good enough that they are capable of echolocation. It works wonders in the constricted corridors and halls of the Labyrinth, but works significantly less good in open spaces (another reason why they are less than happy to leave Labyrinth for extended periods of time). The eyesight plays a supportive role for tunnelers, helpful on occasions but simply not something that they are willing to completely rely on.