The Bloom was a hive mind which developed on ERM 4997. It originated from deep beneath the planet's surface and lived as a non-sentient lifeform for thousands of years.
In 2996, IMC established a mining based directly above the Bloom. It fed off the emotions of the miners, slowly growing stronger. When the base was attacked by Sontarans, in the year 3076, the Bloom feasted on the enormous amounts of fear. The Sontaran's aggression was absorbed by the Bloom, making it more hostile.
When the Seventh Doctor and Melanie Bush arrived in the base, the Bloom was strong enough to absorb Tethneka. It proceeded to absorb every single living thing in the base, only the Doctor, Mel, and Ketch escaped.
In order to give the Bloom a second chance, the Doctor destroyed it, turning it back into mineral dust. The dust formed in a creature with the best aspects of the species it had absorbed. This new Bloom was part mineral, part animal, and part planet. (AUDIO: Terror of the Sontarans)