Publisher's blurb: The Cavern-Tunnel Wye (large cavern) group of Naked Geomorphs contains 18 tiles, each with three connected openings, one or two of which are three-or-more position openings (CSD, CSSD, etc.) and the remainder tunnel openings (T). Sets A and B are completely interchangeable. I.e., tile M58A can be replaced with tile M58B, without changing the basic structure of your map. Set B has broad, unrestricted passages.