Humans are prevalent in both the Overland and the Underland. They have been present in the Underland for several centuries, and as such, have adapted to life in a world with no sunlight. This includes pale skin, through which their veins can easily be seen, violet eyes, and Silver hair.
They are known by other species in the Underland as Killers, though they are not called this directly, but rather, this is what the other species call them in their own tongues. The reason behind this is that when Sandwich first went down to the Underland, he came upon a plot of land that he desired to build a city on. Unfortunately, this land was already occupied by a species called the Diggers. They fought a bloody war over this land, which came to an end when Sandwich poisoned the Digger's water supply. This was seen as a dirty and disrespectful tactic, and earned the Humans the nickname Killers for centuries to come.
Once the humans had established themselves on the plot of land, they dug a magnificent city out of the stone, complete with a palace from which a royal family would rule. This city was Regalia, and was described as being extremely ornate, having designs carved into the stone on nearly every structure. They thrived there, creating alliances with some creatures, such as the Flyers, Crawlers, and limited alliances with the Spinners.
As time went on, another city was established elsewhere. This was the Fount, and it was governed by relatives of the Royal family. Both cities were largely agricultural based.
The Underlanders have been at war for centuries with the Gnawers, and as such, have built a very much military oriented style of civilization. This is offset by the fact that they continue to use swords and shields as their primary weapons. Large walls surround the city on all sides, making it very difficult to attack. Because the fields are inside these walls, the city is virtually impregnable, and is able to withstand a siege for as long as it needs to.
Very rarely, a person from the Overland will fall down to the Underland, though according to several people, they generally don't survive for very long.