Lowerdark

Ten miles or more below the surface, the Lowerdark is where even those who knew the Underdark were loath to go. It remains largely unknown to most of the surface races, and houses horrors unimaginable to even many residents of the upper levels. Truly alien societies thrive there with cultures wholly incomprehensible to surface dwellers.

Mapping

Unlike our own world, the center of The Lost Lands is mostly rock, though pockets of magma are not uncommon. The entire region is far too honeycombed and three dimensional to map effectively, and the constantly shifting caverns by tectonic activity or reforming by underdark inhabitants makes mapping it futile.
The regions of the Lowerdark are difficult to reach and are found in their own individual sections.

Inhabitants

While these can be found in the other reaches of the underdark or even on the surface, the true terrors of the world dwell deep inside the world. The dreaded beholders have a vast city at the center of the world, aboleths slumber in ancient underground lakes, elder brains and cities of Mindflayers, terrifying communities of cloakers, and portals to realms inimaginable corrupt pockets of the lowerdark.

Location under

The Underdark