Time field

The Cracks were a series of splinters in the "skin of the universe," according to Prisoner Zero. The pattern of the Cracks is always the same, and the cause remains unknown. When discussing the cracks with Prisoner Zero, the Doctor stated that somebody must have opened the cracks. When he was asked if he knew what the cause was, the Doctor remained silent. (DW: The Eleventh Hour)

List of Cracks

 * The first official crack was seen on Amy Pond's bedroom wall, which soon opened up to another area in the universe - an Atraxi prison. (DW: The Eleventh Hour)
 * The crack was seen on the TARDIS monitor, which the Doctor looked at briefly before switching it off. (DW: The Eleventh Hour)
 * Another was present within the hull of the Starship UK, a space ship inhabited by the residents of the United Kingdom. (DW: The Beast Below)
 * The crack was seen in one of the Cabinet War-room(s) that the Prime Minister worked in during WWII. (DW: Victory of the Daleks)
 * Another crack is to be found in a wall on the inside of the Byzantium (this is the second crack to be seen up close by the Doctor and Amy, who found this "extremely, very not good"). (DW: Flesh and Stone) A teaser for this episode revealed that the nature of the cracks will be explored, and refers to what the crack "wants" as if itself it an entity.