Steel Trapdoor

From the Super Mario Wiki

Steel Trapdoors are the counterparts of Wooden Trapdoors in Wario World.

Unlike Wooden Trapdoors, which could be entered with a Ground Pound, Steel Trapdoors required a Piledriver to get through. Once inside, Wario would find himself in an expansive obstacle course, with no solid ground at all. Falling off the floating platforms would force him to start over. At the end of the obstacle course a Red Diamond would always be found. Sometimes, a Golden Statue Piece would be there as well.