They are cracks in the skin of the universe, caused by an explosion which happened on the 26th of June 2010. The size varies. Rosanna, the leader of the Saturnynians in Venice, said that back in Saturnyn, their planet, they could see people and places through some of the cracks and silence and the end of all things in others. The cracks seem to be following the Doctor and Amy through time and space. There was one in Amy's bedroom,one on the TARDIS's scanner,one on the side of Starship UK, one in the London Cabinet War Rooms in 1941, one on a wall in the Byzantium and one in the sky of Venice. There are two types of cracks: the first is pure time energy and it can erase and remove things completely from time and space. This is happened to the 2009 Dalek Invasion of Earth and the Cybermen invasion in 1851 London and the Weeping Angels on Byzantium. Only time travellers can remember these events. The second type is a passageway between two places in time and space.