The Underground is an expansive series of catacombs below the surface. Long ago these were the cities of mankind before the Tabernacle arrived. Afterward mankind fled below the surface and lived for three hundred years in darkness, digging out new cities and roads while doing their best to avoid the brunt of the mutation rain. When the rains died down mankind returned to the surface and gradually forgot or abandoned the subterranean cities.
Most of these cities are off-limits or forbidden for entry, but that doesn’t stop anyone willing to do a bit of digging and exploration. Monsters have moved in and live along most of the old roads, while criminals, gunslingers and revolutionaries eke out an existence deep underground. There is treasure to be made for those who seek them, and the underground is safe from the prying eyes of Lawful factions. It’s a lawless place where only the strong survive, but survive they do.
When planning an adventure into the underground, keep in mind that the ruins of 1997 cities are heavily overgrown and almost undetectable save for the very-very rough remains of buildings and statues. GoHyaku takes place around 6,500 AD. That means the time between 1997 and 6,500 is about the same amount of time between 1997 and the construction of Stonehenge. Most of the “lost technology” the Gunslingers and adventurers will find will be from around the 50th century AD, when people began leaving the underground, and magic was still in its infancy. This means that treasures found in underground catacombs can be magical in nature, or of a size to fit gnomes, dwarves and other races of different builds.
That said, artifacts from the 20th century would exist, somewhere, and be incredibly rare and virtually priceless. This could make a strong adventure hook for say, a gunslinger or a monk.