HistoryEdit, located in a cave behind the entry point to the age.The Age was originally written by Gehn, during his attempts to escape his "prison" on Riven, but proved, much like his other ages, to be unstable. He cast it into the fire to burn, but it was salvaged by the Moiety.
While Katran stayed with the Moiety in their caves as their leader, they told her about the book, which, using her incredible writing ability, she was able to rewrite it as a stable age to house the Moiety safely from Gehn.
In order to link to Tay, the Moiety had to discover the combination that operate Gehn's domes to power it, and steal a Riven linking book from Gehn as a means to return. Catherine made sure to fix the Linking panel by a kind of special book-powering crystals.
Once the Moietior-Esk gave word that Gehn powered to use the domes, Catherine used a dome to power the damaged linking book, linked to the Moiety Age to locate the substance, which she refined or adapted it to form book windows, that would heal the books. Thus she was able to link back to Riven, with more book windows. Soon Catherine managed to fulfill the "prophecies" and all the Moiety were led to the new Age and they named it Tay. To preserve their security, Catherine reluctantly considered destroying the links between Tay and Riven so that Gehn wouldn't discover them, but she knew that her companions wouldn't like the idea of severing the connections to their original home.
When visiting the age, the Stranger linked to a cave. He was captured by two Moiety guards and brought to a closed quarters. There Nelah provided him with the Trap Book they had stolen from him, as well as a linking book back to Riven.
Catherine relocated the Rivenese to Tay to save them from the imminent collapse of the age. Presumably she carried the linking book to Tay when she linked back to Myst (although this is not seen in the ending video of the game).
All that can be seen of Tay is a deep valley occupied by a vast lake surrounded by encircling mountains. Around the walls there is at least one cave which is the link-in point, and also has a Linking Book to Riven. The Moiety had built an idol of Gehn, which is superstitiously pinned with Rivenese Daggers.
The main feature of the Age is a massive tree by the lake (perhaps written as a reminiscence of the Great Tree of Riven), on which a big hive-like structure was built. The Hive has inside several rooms where the Moiety dwell.