This article is about the programming term. For the location, see Grim Underworld.
The Underworld is a programming term used by Jagex to describe the area visible beneath the surface of water in RuneScape.  The Underworld was developed as part of RuneScape High Detail, which included a redesign of the game's water mechanics.
Everything visible beneath a water surface is programmed within The Underworld. Objects such as reeds, boats, rocks, and shorelines are some of the most frequent examples, although many others exist.
Prior to the addition of RuneScape High Detail, water surfaces were presented as repeated textures, with no actual motion or depth. Plans to change the water mechanics of RuneScape existed for some time prior to RuneScape High Detail, but a number of ideas were considered at length before the development of The Underworld was settled upon.
Map squares typically have five levels, with the surface usually being level 0 and roofs usually being level 3. Most "underground" areas and dungeons are actually on level 0, but a long distance to the north of the rest of the world (this can be observed using map markers). The Underworld is located on level -1, so as not to interfere with the game's teleportation programming (which usually places players on level 0).