Big globey thing

A device which the Eleventh Doctor described appreciatively as "big globey thing" sat in the centre of Walter Simeon's office at the Great Intelligence Institute, being home to the newborn Great Intelligence's physical substrate. It was, according to Simeon, "highly valuable equipment".

Nature
The device was used to house the intelligent core of a flock of telepathic space crystals, which Simeon had discovered in his garden as a child, only for it to imprint on him, reflecting his darker impulses and gaining sentience. The huge device sparked with electricity and the snowflakes inside were constantly fizzing about, becoming denser and more active when the growing intelligence was roused. (TV: The Snowmen)

History
The "big globey thing" was built by Walter Simeon at some point prior to December 1892. He would consult the consciousness on matters of strategic importance while being its eyes and ears in the wider world. After the Eleventh Doctor forced his way into the Institute, posing as Sherlock Holmes, he violently hit the globe with his walking stick to wake up the being, which identified itself as the Intelligence. After making a few deductions, the Doctor left.

Some time later, the Doctor returned to Simeon's office and tricked Simeon into touching a memory worm, erasing all of his memories. He hoped that this would render the space crystals inactive, as they'd no longer have anything to reflect. Instead, this allowed the Intelligence to leave its previous physical embodiment and fill Simeon's body. After Simeon died, the Great Intelligence left his body but did not return to the globe, having learned to exist as a fully disembodied consciousness. (TV: The Snowmen)

Behind the scenes

 * The antecedent to what became clear, tubular casing of the Prime Minister of the Daleks in the final draft of TV: Asylum of the Daleks was a large crystal globe inhabited by the Dalek Prime.