The Source (The Doctor's Daughter)

The Source was a terraforming device used in the colonisation effort of the planet Messaline. Over the time that the human and Hath colonists fought each other, the Source's true purpose was forgotten. The humans, at least, believed that it was the "Breath of Life", breathed by a female Creator at the beginning of time and captured. The two races also believed that it had the power to destroy the race opposing the one who held it. It turned out to be the device their ancestors had brought to the planet to terraform it.

It was located by the Tenth Doctor. He broke its casing to activate it, showing the two races the true purpose of the Source and the reason they were actually on the planet. (TV: The Doctor's Daughter)

Behind the scenes
The Source showed that it really was the "Breath of Life" since it revived Jenny after she was shot and killed by General Cobb. (REF: The Time Traveller's Almanac)