Engine Room

From the Super Mario Wiki

The Engine Room is a Dome in Super Mario Galaxy. It is the fifth accessible Dome, and will be unlocked whenever Mario recovers the Grand Star from Bowser's Dark Matter Plant.

On the exterior, the Engine Room looks simply like a large metal dome. Inside, however, are purple and blue pipes leading up from a steel mesh floor. A Gearmo patrols the room, although he serves no real signifigance. As with all Domes, the Engine Room has a Pull Star in the center, which is used to access the Dome's Galaxies.

The main boss of the Engine room is King Kaliente.

The Areas of Super Mario Galaxy
Domes
Terrace | Fountain | Kitchen | Bedroom | Engine Room | Garden
Other Areas
Comet Observatory | Garage | Library | Trial Galaxies | Gate
Other Galaxies
Bowser's Galaxy Reactor | Gateway Galaxy | Grand Finale Galaxy

The Engine Room
Gold Leaf Galaxy | Sea Slide Galaxy | Toy Time Galaxy | Bonefin Galaxy | Bowser Jr.'s Lava Reactor | Sand Spiral Galaxy