THE D'NI PEOPLE
Roughly ten thousand years ago, the people who have become known as the D'ni were known by another name and lived on another world. This group of people left their original home world because it would soon become uninhabitable. They wanted a chance to begin again.
They possessed a technology, known as The Art, which allowed them to travel to distant branches of what they called "the Great Tree of Possibilities" by carefully describing the place where they wanted to go in a special Book.
When this rather small group escaped from the destruction of their home world, the description of the place that they wanted to go brought them to a cavern deep below the surface of the Earth.