Undergrunts are enemies in Super Mario Galaxy and Super Mario Galaxy 2. They are a large sub-species of Monty Moles. They are large moles with dark brown fur, light brown bellies, cheeks and noses, and a large green and yellow spiked helmet. Since these enemies dig in the ground on certain planets (usually complained by Star Bunnies), this is the only indication of an Undergrunt, besides the fact that they leave large dirt trails while they dig. Undergrunts appear as rare enemies in the Gusty Garden Galaxy. They will constantly dig through the planets. Mario/Luigi cannot do anything to them while they are digging through the planets, and trying to jump on their helmets or being run over by one will cause them to lose a wedge of health from their Health Meter. Undergrunts can only be stopped if the player Ground Pounds near a digging Undergrunt, the Undergrunt will flip over and become stunned for a few seconds. Mario/Luigi can then Spin or jump on them to defeat them. If they are Spun at this time, three Star Bits are rewarded to the player. If they are jumped on at this time, a Coin is rewarded instead. Undergrunts also appear in the Gold Leaf Galaxy. Apart from Undergrunts, a boss in the Gusty Garden Galaxy is a giant Undergrunt named Major Burrows. Also, there are Undergrunts that control large cannons and serve as bosses in Super Mario Galaxy and enemies in Super Mario Galaxy 2 known as Undergrunt Gunners. In Super Mario Galaxy 2, Undergrunts remain as rare enemies. They appear in the Battle Belt Galaxy. They must be defeated to proceed. Undergrunts behave the same as they did in the prequel. In addition, Major Burrows appears in the Boss Blitz Galaxy as a boss Mario/Luigi must face, and Undergrunt Gunners appear as somewhat common enemies that appear in a few galaxies, such as the Fleet Glide Galaxy.
- The names of Undergrunts are based off of military terms. The "grunt" in Undergrunt comes from the same word used in military to mean a low ranked soldier (much like how Undergrunts are one of the many minions of Bowser). The "Major" in Major Burrows and "Gunner" in Undergrunt Gunners are also military terms.