B1 "In Search of the Unknown" by Mike Carr was TSR's eighth "Dungeon Module" (following close upon the heels of G1-3, D1-3, and S1, all published in 1978). The introductory adventure replaced Dungeon Geomorphs in the Basic Set and was also sold separately.
Full-page advertisement for module B1 "In Search of the Unknown" appearing in The Dragon #22 (February, 1979) and #23 (March, 1979)
"In Search of the Unknown" has been discussed extensively on internet blogs and forums (see the Dragonsfoot index, here). One fan constructed an impressive 28mm scale model of the first level. The module has even inspired a rock ballad.
According to an interview with Mike Carr on the Save or Die podcast (December 4, 2010), the background wasn't based on any pre-existing adventure or campaign setting, but rather invented out of whole cloth, an exercise in rational dungeon design.*
*the construction of underground fortresses is a logical extension of a world in which magic actually works, as outlined by Lewis Pulsipher in his article "Castles in the Air: Why dungeons exist" published in White Dwarf #42 (June, 1983)
Pregenerated characters from the "monochrome" version of module B1 "In Search of the Unknown". Note elven fighting men and magic-users, as well as demi-human thieves, in keeping with OD&D as opposed to Holmes Basic.
The pair of magic mouth spells in the entrance alcoves are similar to another pair of magic mouth spells at the entrance to the mini-dungeon "F'Chelrak's Tomb" by Jennell Jacquays, published in The Dungeoneer #1 (June, 1976), which utter the warning:
"Go back Foolish one or meet thy doom. Go back, go back, go back, go back..."
(Most recently, I've been working on a version of the adventure in which the PCs travel back in time to when Rogahn and Zelligar still occupied their fortress, and are compelled to accompany the famed duo on their ill-fated foray to barbarian lands...)
The "In Search of the Unknown" Campaign Sourcebook (2009). A collaborative effort by fans of module B1.