Prison Island was the smallest of Riven's five islands. After the Age broke up into different land masses, this island drifted far away from the other four until it could no longer be seen from any of them.
The island was once the site of the Great Tree of Riven, which was the tallest in all the Age by a wide margin. However in his quest to produce functional linking books, Gehn had the tree chopped down and converted into paper. By the time of Riven's final days, only the massive stump of the tree remained. It dominated the island, leaving only a few outcroppings of rock visible. Gehn built a prison cell on top of the stump, in which he imprisoned Catherine, leader of the Moiety. The capture of Catherine was an attempt to limit the Moiety's power, and perhaps be a bargaining chip with Atrus, should he ever return to Riven. When Catherine was eventually freed by the Stranger, she immediately set about evacuating the Rivenese to Tay.
The island's remoteness made it an ideal prison. It was impractical to connect it to the other islands via bridge or maglev, so it could only be accessed by the linking book kept in Age 233. The island's firemarble dome was built just off the coast in the shallow waters surrounding the island, and it marked the only way to leave.