This is an underground city that can be reached by taking a passageway behind one of the grand waterfalls. The city is build in a fashion that is wall-to-wall. Since the cave is only barely tall enough for a single story building, the buildings are made to be the floor being the bottom of the cave and the roof being the top. They are made of strong stone. Many people that live here sell fossils of various common pokémon that used to live in the caves and currently live in the vast waters outside.
The gym is close to the city, deep in the network of caves. In order to find it, you'll need to follow the wooden signs that lead you to the right spot. The building is a metal structure. You will battle three gym leaders.