Not Actually Fiber Optics, but Light Pipes
I have also loved the idea of a cave. Or even an underground complex with several family units. I don’t mean to come off as a know it all as I am not (by far, LOL). However the sunlight needed to keep our bodies in rhythm with the seasons and to provide light for seeing and for growing plant life. Would be supplied by Light-Pipes. A sort of big brother to fiber optics. Fiber optics are used for data communication and for entertainment or ambient lighting effects. However a Light-Pipe is quite different. Picture a 12 inch round dome on top of the given mountain or mound. This would collect light. The light pipe fastened to it. Would look much like a clothes dryer output pipe. Except the inside would be coated with a highly reflective material. Carrying the light down to a diffuser of almost any shape needed. This type of Light-pipe is used today in homes and the largest installation I have seen was in the Canadian Eaton Centre Mall in Toronto. Here one larger collector was connected to many light-pipes to carry light throughout the structure. This was several years ago and I do not know if the system is still in place. See actually I am wrong they are called light tubes and can be looked up on Wikipedia at this link ( http://en.wikipedia.org/wiki/Light_tube ). Well that’s my 5 cents I hope to invite you to my cave for a beverage some day: nydlstrick….