Morgo the Mighty takes place in a series of massive caverns located beneath the Himalayas. Each cavern is lit by a different degree and spectrum of light. The deeper one goes into the caves the brighter the light becomes. The explanation of where the light comes from is a bit ridiculous but probably no more so than the idea of a series of massive caves, teeming with life, located beneath the Himalayas.
If I ever do manage to do a rewritten and illustrated version of the story I plan to have the light originate from a different source than in the original novel. That source will probably be no less ridiculous than the source in the original.