Some volcanic, some natural, and others carved out by a mixture of animal and cybrii hands, the island is criss-crossed by vast networks of tunnels. The largest of these networks became a trade highway between the other three villages.
These main tunnels sit near the surface, and connect at a giant cavern just north of Mount Fulkana. Elder Onok was the first to discover the place in his early days, before the Elders were tamed and the first Cybrii walked the island. It grew from a springfed hole into a home, and from a home into a city.
These days, "The Underworld" is home to not just Elder Onok and the Umbrii that share his element, but Cybrii of all elements as well. Most notably, Umbra boasts the largest single population of hot-headed Ignii. The mix of cultures leads to frequent problems for the Elder to resolve. His long friendship with the Igni Elder, Hakhoz, is likely the only reason the city has avoided collapse after all this time.
The city is split into three mostly-flat platforms by an ancient limestone buildup, each spilling out of the base of the next one. When the Ignii sought refuge after failing to build a city of their own, the Umbrii granted them the lowest two tiers of the cavern. Only the topmost tier of the cavern connects to the tunnel networks. Due to their unpleasant temperment and "diluted" element, Ignii are barred from the city's upper tier except by permit.
The ban is just one of many reasons for the high tensions between the city. "Thrown down to burn with the rest of the trash" is an unfortunately common term among the city's upper crust.