The Iron Worm is a mini-boss found near the end of the first area of Aqua Tunnel. It is a large mechanical worm-like robot whose head resembles a deep-sea diving helmet.
The Iron Worm opens it's hatch to shoot damaging green blobs at the Player Character. Milla can use four Phantom Blocks to defeat this mini-boss. Once done, the machine explodes, and its head lands onto a switch that opens up a path to the second area.