A flume was a portal that Travelers used to move between the territories. The flumes were created by Saint Dane to allow him to mix the territories, though the travelers also used them in their mission to keep the territories on their separate paths. They were later destroyed for that reason.
Flumes are strategically placed in abandoned areas, so that regular people would not stumble across them. They are marked by a crude, engraved star, so that Travelers can easily identify them. When accessed by anyone, they may have to avoid the quigs, yellow-eyed beasts that were created by Saint Dane to trouble the Travelers. The quigs were unique to each territory.
The Subway in the Bronx
One of the more iconic flumes throughout the story, this hidden flume in an abandoned subway station in New York starts off Bobby Pendragon's grand adventure in The Merchant of Death. During many occasions in the early titles, Bobby revisits this flume. In The Never War, this flume's surrounding appearance changes dramatically. On First Earth, The subway is built as very snazzy and appealing new subway in 1937. On Second Earth, however, is the abandoned subway mentioned before. But on Third Earth, it is an astounding underground super mall built into the Bronx as seen only in The Never War.
This abandoned mansion in Bobby Pendragon's hometown of Stony Brook of Second Earth was home to a large family a long time ago. The family quote: "...made a living of raising and selling chickens, of all things..." soon lost the business and left the home. In The Reality Bug, Mark Dimond & Courtney Chetwynde trespass the home after their confrontation with Officer Wilson after Mark's first attempt. Later on, while running from Quigs in this house, Mark's ring creates the flume during a scene where they run threw a hidden room with curtains.
The flume can be destroyed and crumble with the continued use by Acolytes or regular inhabitants of territories in Halla. Its destruction on Eelong led to the death of Kasha in Black Water and the confinement of Gunny and Vo Spader. This was due to the fact that Mark and Courtney used the flume to deliver the antidote to the Cloran poison from Cloral. However, these rules did not apply after Saint Dane's Convergence, when all were free to travel throughout Halla.