Mount Ossa
Just as Mount Olympus dominates much of Arborea’s first layer, so Ossa represents its intrusion into the other layers. Mount Ossa is an extinct volcano, truly enormous, but also continues down under the surface of the sea of Ossa. One side of the volcano is occupied by Tlalocan, the Aztec paradise. Storm clouds mass permanently around the top of the mountain, and none has ever entered the crater and returned. To travel to the rest of Olympus, one must travel underwater, for there are no Olympian portals above the surface. Tritons and nereids guard the ways, and only allow those to pass who can amuse them or provide some service.
Source: Alex Roberts