The River (F-Tunnel) is a tunnel in Run 3: Explore Mode. It is unlocked after completing the M-Tunnel.
Once the Ringed Box sidequest is activated, one of the boxes can be found in The River, part 3.
All the levels in The River were made by Precarious, who originally named the level set "Riptide River Rapids". They were implemented into the main story on [date].
The following is a brief description of the story in The River (F-Tunnel). More information on the story can be found here.
After beating M-5, the cutscene 'Insanity' will begin, in which the Duplicator is watching the Runner's attempts to reach a tunnel in the distance. The Duplicator asks the Runner if she knows what they say about going insane, because he thinks that the Runner is doing the same thing again and again while expecting a different outcome. The Runner responds that she is not doing the same thing before tripping off the edge. When she respawns, the Runner notices that the Duplicator drew on her map. He tells the Runner that it is a picture of her to "commemorate this moment" so they'll remember when she went crazy. The Runner is surprised, but she ignores it and jumps again, this time successfully reaching the next tunnel (F-Tunnel). After the cutscene is finished, the player will start F-1 with whichever character they used last.
After beating F-3, the cutscene 'River' will begin, starring the Duplicator and Child. The Child says that he calls the F-Tunnel "The River" and goes on to explain that a river is a"thin blue line on the surface of the planet." He continues saying that rivers are really wavy, like the F-Tunnel, and he saw one in a book. The Duplicator reminds the Child that books are fiction.
After beating F-5, the cutscene 'Socratic Method" will begin, in which the Runner agrees to let the Child change the name of the F-tunnel, but nothing else. She goes on to explain why, showing him that if, for example, she moved the position of level A-1 to the end of the Box Storage Area, the map would be wrong because there's nothing there. The Child understands, so the Runner allows him to change the name from "F-Tunnel" to "The River". After this cutscene, the F-Tunnel is changed to "The River" on the map.