Skull Boxes are black artillery that first appeared in Super Mario World. They fire Torpedo Teds at Mario. Much like Bill Blasters, the Skull Box cannot be destroyed.
History
Super Mario series
Super Mario World
Skull Box first appeared in Super Mario World in only one level, the underwater level, the Soda Lake where they fire Torpedo Teds they are invincible so they cannot be defeated by anything. This makes the level tricky to traverse.
Super Mario Galaxy
Skull Box also appear in Super Mario Galaxy, but they look different and are called Ted Blasters. They are most likely found in the water, hence the name Torpedo Ted. Also in this game they actually can be destroyed by leading a Torpedo Ted right back at them.
New Super Mario Bros. U
Skull Boxes, as well as the Torpedo Teds, will return in the upcoming Wii U game, New Super Mario Bros. U. They do behave and look exactly like in Super Mario World. They are now seen in Larry's Torpedo Castle. Unlike Super Mario Galaxy, where they could be destroyed, in this game they are invincible. Missile Torpedo Teds and red Skull Boxes debut in The Mighty Cannonship level, being both obstacles and the weapon to destroy Bowser Jr..
Super Princess Peach
Skull Box return alongside the Torpedo Teds in the game Super Princess Peach where they are still invincible.