Diet Cola Mountain is a soda bottle shaped volcano with a pit of boiling hot Diet Cola instead of lava. There are Mentos stalactites that sometimes fall from the roof of the volcano, causing fizzy eruptions! Diet Cola Mountain was home to Vanellope, who lived in a secret un-programmed hideaway inside the mountain (by entering through two sugar free lollipops at its base) before she moved into King Candy's former castle. There is an incomplete racetrack within the mountain, suggesting that the mountain was intended to be a bonus level, but the game creators never got around to finishing it; this is loosely confirmed by Vanellope, as she is only guessing it is true.
After breaking into King Candy's kart factory to build a kart, Ralph and Vanellope are pursued by the king and his henchmen. They manage to evade them by escaping to Diet Cola Mountain, where Vanellope introduces Ralph to her shabby home. After learning more about Vanellope and her motivation to race, Ralph creates a racetrack inside the mountain to teach her how to drive.
Later on, Sugar Rush is taken over by Cy-Bugs—one of which eats King Candy, turning him into a cybernetic monster bent on taking over the arcade. Ralph formulates a plan to erupt the mountain and use the lava as a beacon to defeat the monsters. At the mountain's summit, he is confronted by King Candy. The two battle, and King Candy lifts Ralph high above the mountain. Ralph breaks free, and the impact of his fall erupts the mountain, destroying Vanellope's home. As planned, the lava attracts the Cy-Bugs and King Candy to their deaths, saving the arcade.
- Diet Cola Mountain could be a reference to gaming in real life; Diet Cola Mountain is supposed to be a scrapped level, but Vanellope managed to make a home out of it. In real life, people can use cheats and hacks to access elements in video games that were scrapped or left unfinished.
- It is also based on a popular internet meme involving an explosion in a form of a fountain caused by Mentos being dropped in Diet Coke. The "Bit by Bit" feature about the making of Wreck-It Ralph, which can be found in the Blu-Ray extras (sadly not on the DVD), shows the production team actually trying this experiment themselves.