The Blessing

The Blessing was the name given to a pole of the Earth, going from Buenos Aires to Shanghai. According to Captain Jack Harkness, it was alive. (TV: The Blood Line)

Origin
Despite taking charge of the Blessing, the Three Families were unsure of its nature, such as whether or not it was a living organism. When pressed by Gwen, Jack speculated that the Blessing was something spoken of in a Silurian mythology he had heard of from the Doctor, and could have been caused by an interaction with the huon particles of the Racnoss hibernation field, but was forced to admit he had no idea what had created it. (TV: The Gathering, The Blood Line)

Miracle Day
The Three Families used it to cause the miracle, by feeding it Jack's immortal blood. Merely being on the street close to the site of the Blessing caused people to feel uncomfortable and haunted, and seeing it was said to show people their true selves. (TV: The Blood Line) In some cases, this was known to drive some people, such as Zheng Yibao, to suicide. According to the Mother, these people were always hiding "something vile". (TV: The Middle Men, The Gathering)

The Three Families discovered the Blessing's presence by realising that the average lifespan of people living in Shanghai over the Blessing was the exact same as the global average human lifespan, and had been so throughout history. The Blessing was the focus of a morphic field matrix that encompassed all humankind, and the Families were able to alter it by introducing Jack's blood on both sides of the fissure. Jack speculated that the Blessing perceived this as an attack, which is why it was trying to keep everyone alive. As a mortal, Jack's blood was attracted to the Blessing. Introducing his mortal blood on both sides, via Jack himself and Rex Matheson, reversed the process. It was later sealed off by UNIT to prevent any further manipulation. (TV: The Gathering, The Blood Line)

The Three Families were unable to repeat the process because Holly Mokri had already destroyed the Key, the facility housing extra stores of Jack's immortal blood. (WC: Web of Lies)

Alternate timeline
The Blessing was visited by John Hart, who had made himself ruler of the British Empire in his own alternate timeline. He walked away commenting that it looked like "a giant arse". (AUDIO: The Death of Captain Jack)