From the Super Mario Wiki
The Subspace Bomb is a giant time bomb appearing in the Subspace Emissary in Super Smash Bros. Brawl. The Subspace Army uses this bomb to take the world into Subspace. In the game's introduction movie, one Subspace Bomb is dropped into Midair Stadium by the Ancient Minister. The bomb needs R.O.B.s on either side of it to detonate. Once activated, it will start the three-minute detonation sequence and when time runs out, it will explode.
Subspace Bombs are used in various parts of this world, such as the Midair Stadium and King Dedede's Castle. When Tabuu invades the Island of the Ancients and puts all of the R.O.B.s to work, he converts the island into the Subspace Bomb Factory and puts Ganondorf in charge. Eventually, all of the heroes reach Tabuu in Subspace and defeat him. As the Island of the Ancients is pulled into Subspace, the production of Subspace Bombs is ended, and the Island of the Ancients can be seen as a gleaming 'X' in the sky.