There are still natural sugars (from fruits and other flavorings), but the product is mostly made from sugar substitutes, such as Aspartame and/or Splenda.
MythBusters concluded that the potassium benzoate, aspartame, and CO2 gas contained in the Diet Coke, in combination with the gelatin and gum arabic ingredients of the Mentos, all contribute to the formation of the foam.
Despite the sparse and diminishing availability of the brand, this is the only soda currently recommended by Atkins for low-carb dieters because it does not use sugar or aspartame.