Okay, so Dylann is the name for this huge, underground cavern. At one point in time it was heavily populated. There's a river that runs through it and people had ponds stocked with fish and they grew... other stuff. And the way they existed underground was from the light produced by the crystal trees.
The trees come in all colors and sizes. Some forests were green, some were blue, some were white. They glowed, of course, and were slightly warm to the touch. Besides being the lightsource of the entire cavern the trees were also a sort of cobha conduit. Collectively the forests are not quite sentient, but empathetic living organisms. They can sense the desires and the emotions of the people around them and can often respond to that in some way.
For example, if a child is afraid of the dark the trees will glow more brightly. If someone is lost they could point a path. The trees also sing. I haven't exactly worked out anything more detailed than that, I just know that they do. (Like wind rustling through leaves, except there's no leaves and there's no wind...)
Questions? Comments? The forests are one of my favorite aspects of Dylann and, like everything else in my life, Highly Undeveloped.
The sentient bit is very intriguing and useful for making emotional scenes.
What a beautiful idea! It's amazing! I love the ideas of glowing crystal trees! Hey, are they salt crystals? 
That would be handy. 
