Far below the sunlit surface is a world of dark and twisting caves. Within this world is a particularly dangerous delve known as the Lost Underground Galleries. These caves might exist by themselves or they might be part of a larger whole. They might even comprise a sinister sub-level situated off of the main core of an inordinately large dungeon complex. It's up to you to figure out how to place this locale within your campaign.
There are two ways into these galleries, each denoted on the map by a dotted circle. Where these access points lead is up to you. Perhaps they connect to the surface. Then again, they might lead to yet deeper caves—or even to the fabled caverns which are said to honeycomb the crust of the world.
Who or what dwells (or dwelled?) within these dark caverns? There has obviously been some sort of intelligence in this fell place due to the presence of several unusually large statues. Are the builders still here or did they move on centuries ago? Is anything crawling out of all those sinkholes? Where do they lead?
If you look closely you will note that there are a number of cysts and vaults that are inaccessible from the main series of caves. This is by design. Note that these cysts each have their own individual access points. Again, whether these access points lead upwards or downwards is up to you. Perhaps they all interconnect via some sort of arrangement of hidden passages. There are many possibilities.
The Fine Print: I am sharing this map under a Creative Commons Attribution-Noncommercial-Share Alike license. If you follow that link you will be able to read about the conditions that apply to this work. In a nutshell: (a) you can't use it commercially, (b) you must attribute it to me, and (c) you must share any derivative works that you create.