The Nexus book machine
The Nexus (also known as MT, short for Mass Transit) refers both to an Age, as well as a machine within it, which is the only notable feature of that Age. The Age was created only to host the machine and the books; the rest of the Age cannot be explored, everything else about it remains unknown.
Primarily, the Nexus was created because the D'ni assumed that linking from one place to another in the same Age was impossible. Travelling the huge distances in the City of D'ni by foot would have been unfeasible, and the architecture of the City was impractical for vehicles.
The Nexus acts as a proxy: Nexus terminals (also known as MT stations) throughout the Cavern can register a person's location, authenticated through the KI, and from that moment that person can get back to the same location through the Nexus machine's City Locations list. For unknown reasons, the registration can be undone afterwards.
Since access to KIs was highly restricted at first, it is unclear whether access to Nexus terminals was available to the general public, whether it was added later, or not at all.
The only features are the rotating walls, which house hundreds of Linking Books, and a large KI terminal. Once an explorer places their KI in the terminal, a list of Ages to which they have access appears on a screen. The explorer can browse this list, and choose an Age they wish to Link to. Once this choice is made, the terminal fetches the Linking Book indicated on the screen, and then opens it in front of the explorer. The explorer can then link to the Age they have chosen, or choose a different Age.
The KI remembers any D'ni Ages they have been to, their neighborhood, and any of the Nexus terminals that have been activated with that KI. The machine's interface also provides details such as the current population present in the City and other locations in the Cavern. Additional lists of Private Links, Personal Links and Public Links allow one to link to Ages (not limited to D'ni) explicitly shared by someone else.
After a target location has been selected, the machine will automatically find the appropriate Linking Book, opening it up to reveal the Linking panel.