What's Inside the Mud? (特別なドロの中身, Tokubetsu na Doro no Nakami, lit. Contents of the Special Mud) is an event in Tomba! 2: The Evil Swine Return.
Clean the Mudball Surprise.
Find one of the Mudball Surprise items in the Pipe Area to activate this event. They can be obtained after getting the hammer.
Bring them back to the Washing Machine in the engine room and put it through the wash to clear the event.