This is the second version of Completely Unfathomable, I believe. This is a long-awaited Swords & Wizardry version (so long awaited that S&W is no longer the OSR's favorite system), but a Dungeon Crawl Classics version came out a few years ago.
I have Operation Unfathomable (the below ground version of the setting). It's a deeply weird/gonzo underdark like you've never seen. I don't think it qualifies as a "mythic underworld" unless time travelers, people without heads, hungry bug men and chaos gods show up in real world myth somewhere.