Our world is built on the remnants of the old. Many a farmer has uncovered a ruin when plowing their field. Many a homesteader has uncovered a crypt or a dungeon when laying a foundation for their house. It is common knowledge that there is a world before and that we’re living on top of it. The extent of that world, however, is a subject of debate among scholars and historians. Some believe the last great wars covered over the land and others believe the world simply reclaims the works of thinking races maybe more quickly when there is a taint on the land. Whatever the reason, these lost civilizations did leave behind sewers, old roads, dungeons, crypts, and all manner of underground complex.
In addition to the man made structures underground, there too exists caverns that are vast and largely unexplored. These caverns are usually refereed to as the deep by dwarves and are mined extensively and they turn many of the deeper caverns into beautiful works of civil engineering. These sometimes add to the tunnels and halls already present underground and thereby create vast complexes of varying architecture styles and natural stone. Caves on the surface often lead nowhere but some open into these complexes and still others delve even deeper than the deep.
The Underdark is another world entirely. What is commonly believed about the Underdark is only that it exists. Most think it’s the caves below us. Some think it’s a combination of the constructed and natural. Others believe it’s a euphemism for a crypt or burial site. This means that the Drow hail from tunnels and crypts. One can then understand the animosity or apprehension that many feel towards them. This is not, however, the case. The Underdark is literally another plane. It is accessible through specific cracks and fishers deep underground and while it does connect to the over-world through the deepest places, the Underdark is not truly part of our world but almost an elemental plane of cavern…. if cavern was an element. It’s complicated. Most don’t know this or even understand the possibility of it. Even the residents of the Underdark don’t realize their in a separate dimension and the passage between the Underdark and the over-world’s plane of existence is seamless. Those passing through the barrier don’t seem to notice there is a barrier… just more cavern and cave.
Treasure is another commonly held understanding the over-world has about the world under their feet. Children dream of gold coins in buried tunnels and play pretend in their blanket forts. Adults read fantasy stories and histories about heroes diving into the darkness to defeat horrible creatures and bring back fantastic magical gems. It’s something unattainable but interesting to think about… until you’re plowing your field or laying your home’s foundation and a horde of skeletons emerge.