"Given that we're two or three miles underground, how exactly are we able to see across this vast cavern? The rules say that the light from our torches stops suddenly at 60'…"
"It's moss"
"Moss???"
"Magic … glowing … moss…"
Basic Information
"Magic Moss" is a form of handwavium found in all cave systems and dungeons where the creator requires illumination above and beyond that provided by traditional torches.
This stuff is great for illuminating grand vistas of the subterranean world (as in the flavour text), providing light for underground agriculture and generally for getting light into places where it shouldn't really be.
Despite the name, it needn't necessarily be a plant - glowing minerals and things also fit in the category; if it's designed as a daylight substitute, it's magic moss.
Not to be confused with meat moss.
Sources
Appears in all sorts of places … the classic TSR module D3: Vault of the Drow is a good example of the species, but the ur-example is probably found in the unexplained illuminations of Jules Verne's A Journey to the Center of the Earth1.
Game and Story Use
- Short of outright magical lighting or a truly intimidating set of light transmitters connected to the surface this is probably your only option to light a huge underground space.
- A justification you can use: the Adventurer's Guild was sick of having to bring torches everywhere or being blinded by sudden winds, so they cultivated an aggressively-spreading glowing lichen. There's a bit of an arms race going between monsters trying to find new ways to destroy the stuff and biothaumaturges creating resistant strains.