The Archipelago trial run
Hell, or the abyss. In the beginning, the primordial chaos. The Great Below is under the earth and oceans. In some places, it has pushed very close to the surface. At its depths, molten rocks and frigid waters clash together with nothing to separate them. The Great Below is also home to the Demons.
The places where the great below push through into the world are marked by large amounts of chaos; turmoil, disaster, or eldritch weirdness.