From the White Wolf Wiki, the most complete White Wolf reference

The Unknown Tower' is the name used by some mages to refer to the mythical and mysterious sixth watchtower. Unlike the other watchtowers, the Unknown Tower does not seek out mages; instead a mage must cross the Abyss and search for it before he or she can add their name to its roll.

Most mages are not aware, or do not believe, that the Unknown Tower exists, and those who do debate nearly every detail. It is, however, almost certainly infernal in origin.