The Boomsday Machine is a huge war machine, that looks like a tower, built by Bowser Jr.  He uses it to try to destroy Mario in Bowser Jr.'s Boom Bunker, a galaxy from Super Mario Galaxy 2.


When Mario reaches the Boomsday Machine's Planet, Bowser Jr. brags about his machine, gloats about "making Mario go boom," and starts up his Boomsday Machine.  The tower will try to shoot Mario with lightning balls.  To reach the cockpit at the top of the tower, Mario must become Cloud Mario to climb up to the top of the tower.  Then, Mario must stomp the cockpit to damage the machine.  After that, the Boomsday Machine will activate its vacuums and try to suck in the clouds while Mario is on them to try to make him fall.  Mario needs to climb up to the cockpit again, but must now watch out for the tower's vacuums.  After stomping the cockpit again, Bowser Jr. gets angry and installs tank treads on the machine, and the tower becomes mobile.  Now, not only does Mario have to climb higher to reach the cockpit, but the Boomsday Machine is constantly moving now, and if he is on the ground, Mario must be careful not to get run over.  Once Mario stomps the tower's cockpit for the third time, the Boomsday Machine explodes.  Bowser Jr. survives, but the explosion causes him to be flung deep in space.