This unusual series is comprised of various entries to One Page Dungeon Contests. The "main" scenario, written in 2014 by Mike Monaco, is The Pit. The Pit descends to a Hellmouth. Along the way down, a spiral path traverses thirteen circles. The Helmouth creates portals to fourteen destinations - and each of these destinations is another (winning) entry to the competition (from various years and by various authors).
This intriguing concept thus presents a series of thematically (somewhat) related encounters.
The scenarios, in order from "top to bottom", are:
The Lost Mine
The Tunnels of Turrack the Terrible
Halls of Ksarnia
The Secret Lab of the Mages
Cave of the Stone Sepulchre
Raid on Black Goat Wood
The Crypt of Luân Phiên
The Order of the White Wick
The Halls of the Mad Mage
The Vault of Zerduzan
Watery Palace of the Ooze Behemoth - Level 2: The Ebony Obelisk of the Snail Demon