Hellmouths

There are widely supposed to be nine Hellmouths, or places where dangerous magical energies and the souls of deceased mortals are drawn down into the Underworlds. Hellmouths are also the sites for fiends to emerge into the Prime Material Plane. A few Hellmouths and their general location are common knowledge, though the exact locations of all nine are unknown. Known Hellmouths:

The Hundermaw, in the northern Hundervorne Mountains, is a large crack, from which poisonous smoke and sulphurous fumes arise.
The Caverns of Black Ice are located on the northern shores of the Savage Sea.
The Dread Gyre, a whirlpool of shifting size and location which moves around the islands, exists at the centre of the Ischindar Sea.
The Shivering Sands are located somewhere in the central deserts of Eyl-Thoul.
Another is located beneath the Amethyst Pool in Gyarhame, whose waters are lethal to all life.

Reputed Hellmouths:

A Hellmouth is widely rumoured to be located beneath the city of Mabag Pachamel Grith, though this may be slander by their enemies.
The Dragonborn of Aranbar are sometimes said to worship a Hellmouth located in a dragon's lair.
A Hellmouth is said to exist in the icy waste to the north of the Belphiath.
The twin volcanoes of Mount Morthar and Mound Kharthar are said to be the entrances to a fiery subterranean Hellmouth.
There is rumoured to be a Hellmouth somewhere in the Three Cathaya, guarded by an ancient order of psychic monks.