Tunnelbot, called Gakin (ガキーン) in the Japanese strategy guide, is a sub-boss in Sonic the Hedgehog 3 which you confront at the end of Marble Garden Zone Act 1. It is one of the easiest sub-bosses in the game, made easier by a shield, which resists the falling stalactites. Just like Sonic 2's Drill Eggman II, this boss burrows through the ceiling and back. Hit it whenever you can. With Knuckles, there is a rotating spiked platform that you can stand on to compensate for Knuckles' lower jump height.
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When playing as Sonic & Tails, Sonic can stay at the very left of the boss arena while the second player defeats the boss using Tails. If Sonic is outside of the Act 2 level boundary when Act 2 starts, Sonic 3 alone will crash. Sonic 3 & Knuckles fixes this by forcing the camera, and Sonic with it, to scroll into the overlap between the Acts.