The Underworld is permeated with the great River Nalaganges which connects all of Solcu at various points, across the great continents and even some of it's islands. And it carries with it the Nelumbo.
The Underworld is not reached by the light of the Sun, and so it's denizens must find luminesence in other ways. Some do this through large mushrooms, mountain crystals or magic. Some make use of the Nelumbo flowers.
In truth, Nelumbo is a form of algae which feeds on the light emanated from the powerful crystals which adorn many of the cavern and tunnel walls in the underworld. The algae takes the form of large flower pads, floating on the surface of the water, and comes in many different colours and shapes. When the algae has fed on enough light, it'll bloom into a Nelumbo Flower or Lotus. In order to survive, the Lotus flower will attempt to cast off any excess radiation it might have absorbed - and this has the effect of illuminating the water beneath it. Nelumbo algae can live in both salt and fresh water.
While the Nelumbo can survive in the waters above, the sun will kill the very delicate Deathlight Lotus - thus earning its' name. The flower can not survive very long in direct sunlight, being poisoned by the radiation. Thus, the flower itself blooms only in caverns and tunnels where the river Nalaganges connects to the surface world.
Many surface races scout for luminescent bodies of water at night, knowing that such a pool must be located above an entrance into The Underworld.