Riven, Unknown Space

Atrus and Catherine had previously trapped Gehn on Riven by removing all linking books from that Age; the very last book to be removed, linking to the island of Myst, was the one they held to escape Riven. Atrus hoped it would be destroyed; however, it fell into the Star Fissure, a void leading out of the reality of the damaged Age of Riven and into unknown space. Catherine was later tricked into returning to Riven by her sons, and was captured by Gehn. Subsequently, the Stranger discovered the Myst book, sparking the events of Myst and, some time later, Riven.

Source: Wikipedia > Riven