Titan Depths started as a vertical map for a Map A Day social media event in 2024. Later I had the crazy goal of it being a single place that gamers could spend their entire in-real-life roleplaying exploring, and rather than that be continent or world or multiverse it is an arcology of sewers, tunnels, underground rivers, caves, towns, starless seas, dungeons and megadungeons.

Titan Depths is both a roleplaying game setting and a ruleset. The rules are very much part of the Dungeons & Dragons branch of RPGS, and based on the Basic/Expert/Master ruleset and the Old School Essentials and Old School Essentials Advanced more modern editions .  You could play it without the extra rules  as a straight up OSE game without a huge amount of adjustment.

So, what are the Titan Depths? A vast, t-shaped hole in the ground that many races, civilisations and species have made home. Aberrations from the realms of madness have built a city there. Both Mountain and White dwarves call it home. More than one mad archmage has created huge follies within its expanse. Tunnels and caves teem with life, death and undeath. Underground seas, lakes and rivers provide water and homes for a variety of inhabitants. Magic is common, and mortals can reach almost god-like status if they have the right artifacts. Scattered about the Depths are crystalline sunstones, that provide sunlight and heat. Most have been mined and taken away, but in the furthest places they still grow huge.