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Undergrounders (地底人 Chiteijin) are a race of creatures that appear in all games of the Mr. Driller Series. Although they are not antagonists, they sometimes make a mess of the blocks and food supply. They are considered to be very intelligent, but are scatterbrains. There are 2 races, space and underground undergrounders. In Star Trigon, they are called Uchujin (宇宙人 Uchūjin) and colored yellow, though some sources cite them as "Space Undergrounders".
In all the games, Undergrounds can be found randomly inside certain Blocks, sleeping. If the player wakes up 20 of them, the twentieth will award an extra life.