The Cave(s)* of the Unknown area is left for you to use as a place to devise your own cavern complex or dungeon maze. If you do not wish to undertake this at first, simply DO NOT ALLOW YOUR PLAYERS TO LOCATE IT EVEN IF THEY THOROUGHLY SEARCH THE VERY SPACE IT IS IN. (It was hidden by a magical illusion so as to be undetectable...)
The Keep on the Borderlands
*"Cave" is singular in the Holmes version, and plural in the B/X version, probably as a result of an error introduced while re-typesetting the module
My unkeyed map for the Cave of the Unknown.
I've tried my hand at creating maps for the Cave of the Unknown (one version of which is reproduced, above) but have never gotten around to keying any of them.
The earliest published version of the Cave of the Unknown appeared in "Keep on the Borderlands: The Expansion" (OSRIC, 2013).
Of the other versions posted, I particularly like the one shared by C. Wesley Clough on his WordPress site in 2018, here.
The Caverns of Quasqueton:
Although perhaps not the best fit,* some have substituted the Caverns of Quasqueton from module B1 "In Search of the Unknown" in place of the Cave of the Unknown.
*Rogahn and Zelligar's underground fortress was "far from the nearest settlement, away from traveled routes, and high upon a craggy hill" which is at odds with the nearby Keep and road, (to say nothing of the missing lookout tower)
The left-hand passage has an arrow and the word "QUASQUETON" engraved at eye level only a few feet in; this way once led to the secret fortress of Quasqueton, but the tunnel has completely collapsed; characters can only go this way thirty feet or so before having to turn back.
Return to the Keep on the Borderlands
BEX-1 Descent into the Caves of the Unknown:
Descent into the Caves of the Unknown (2018) by RC Pinnell
BEX-1 "Descent Into The Caves Of The Unknown" by the prolific Thorkhammer (RC Pinnell) is a free, 8 page treatment of the Cave of the Unknown, designed for 4 to 9 characters, levels 3 to 4. The adventure includes a fantastic map, and is quite well done.
B2.5 Caves of the Unknown:
Dungeon Module B2.5 "Caves of the Unknown" (2018) by Charley Phipps, Thom Wilson, Mike Badolato
Druvas shared a version of his map for the Cave of the Unknown on Dragonsfoot, back in 2012, here, which later served as the basis for Dungeon Module B2.5 Caves of the Unknown, (NTRPGcon, 2018).
5eF#15 Cave of the Unknown:
Fifth Edition Fantasy #16 "Cave of the Unknown" (2019) by Michael Curtis
Last, but not least, we have Fifth Edition Fantasy #16: Cave of the Unknown, the most up-to-date of the various incarnations of the Cave of the Unknown.
Designed as a tie-in to OAR#1 "Into The Borderlands". Michael Curtis is an excellent writer and game designer, and this short adventure is no exception.