Published by Wizards of the Coast.
This is one of the very first adventure modules published by TSR, put out about the same time as the G-series, D-series, and Tomb of Horrors. It was created for the Holmes edition of the Basic rules and was included in some versions of that boxed set. There are a few differences between the original pastel covered version and the later brown covered revision that was put out for the Moldvay revision of Dungeons & Dragons. However these differences are minor and really won’t alter game-play significantly.
Read the Full Review at Dragonsfoot
The Holmes Basic Set I owned came with module B1 In Search of the Unknown by Mike Carr, better known as the creator of Fight in the Skies (aka Dawn Patrol). If I had to choose the one module that had the greatest effect on me as a referee, it’s this module, hands down. The reason is quite simple: B1 was written specifically as an “instructional aid for beginning Dungeon Masters” and so it was. I learned a number of really important lessons from using this module — and use it I did — chief among them being this: rooms containing pools of unknown liquids are cool.
Read the Full Retrospective/Review at Grognardia.