The Plane of Earth

“The Underdark? A child’s toybox compared to the twisting labyrinths, caves, passages, and wonders of the Plane of Earth. In the Underdark, there’s an innate sense of the world above that colors the experiences of travelers and inhabitants, but this does not exist on the Plane of Earth. It is suffocation and brutality defined and stands as one of the least hospitable of the Inner Planes. Still, the realm is ripe with treasures and secrets buried beneath the rock and stone.”

--- Astromarchus the Sage

Rock Creatures abound here