When I first got into the whole OSR thing, One of the first websites I found was dragonsfoot.org, and one of the first modules I printed off was the Endless Tunnels of Enlandin. This thing is a UGLY! megadungeon, and my ugly I mean, ieyeyeye it maybe a TPK. Anyhow, this is going into my hex crawl campaign. After re-reading it and checking it a few times, I plan to convert it to Basic Fantasy. It was made for AD&D 1ed, but it can be ran with any of the retro clones no problem. Here's the tagline for the module "Does your group have the nerve to the endless tunnels of the mage Enlandin? Many have tried and all have failed".