The Deepcore is a large underground mine system that is situated under Stoneholm. It begins with the dwarven diamond mines, and ends with the Deepcore Ruins, eventually leading up to the surface. The path through The Deepcore consists mainly of large passages made up of dirt and stone - presumably excavated by the Dwarven Drills - along with interconnecting cave systems which resemble Minecraft's randomly generated tunnels. These tunnels are supported with dozens of wooden pillars.
The Deepcore itself is a large dark, ominous cavern, populated with spiders, zombies, silverfish and various other creatures - seemingly still very much mechanically active hence the abundance of operational redstone circuitry, which may have been triggered by the presence of the heroes.
Xephos and Honeydew (along with Knight Peculier) explored The Deepcore during Minecraft - "Shadow of Israphel" Part 33: The Deepcore, in "a search for answers".