The Unterzee is not dark. The Unterzee glows. You simply have to learn which light to trust.
FROM: Proceedings of the Glimhaven Society of Natural Philosophers, Special Issue BY: Mother Cinder, Priestess of the Luminous Order, Keeper of the Great Sporocarp SUPPLEMENTARY NOTES BY: Commodore Harrowgate (unsolicited but tolerated) The Unterzee produces its own light. This is perhaps the first and most important thing for any surface-dweller (or any young citizen venturing beyond the Upper Galleries for the first time) to understand. There is no sun beneath the stone. There is no moon, no stars, no fire in the traditional sense. And yet the Unterzee is not dark. It glows — in patterns so varied, so beautiful, and so thoroughly resistant to systematic study that I have dedicated my ministry to cataloguing them and have managed, in thirty years, to classify approximately nine percent. The primary source of light is the fungal forests of the Luminous Reaches — vast networks of mycelium that colonise the limestone walls and produce a soft, steady, amber-green glow that the Reach use...
From the lore of The Gaslit Reach.