K37 Gem 5 was the designation of a black hole which served, along with Krop Tor, the planet that orbited it, as a prison for the Beast.
History
K37 Gem 5 and Krop Tor were set up by the Disciples of the Light to imprison the Beast. The Beast was trapped within Krop Tor and a power source was implanted to keep the planet from falling into K37 Gem 5. If the Beast tried to escape, the power source would fail and the planet would be dragged into the black hole.
Over time, legends developed about the mysterious planet and black hole. One, myth, held by the Valtino, described the black hole as a demon. Walker Expedition team member Ida Scott said the mere sight of the black hole tended to drive onlookers insane. (DW: The Impossible Planet/The Satan Pit)
Around the 42nd century, the Second Great and Bountiful Human Empire learned of Krop Tor. The first mission to investigate it, the Galis Expedition, was able to bring back some pottery and a notebook. (TS: Tardisode 8) The next mission, the Walker Expedition, was sent to drill into the planet and get the power source that kept it stable. The Doctor and Rose also came to Krop Tor accidentally. (DW: The Impossible Planet/The Satan Pit)
As the Doctor, Rose and the crew of the Walker Expedition watched, the Scarlet System, home system of the Pallushi fell into K37 Gem 5's gravity field. (DW: The Impossible Planet)
Eventually, the centre of the planet was reached and the Beast was awoken and attempted to escape, possessing Toby Zed and attacking the rest of the expedition with the possessed Ood. The Beast planned to let them escape, allowing his mind in Toby to escape as well. The Doctor confronted the Beast and shut off the power source, pulling Krop Tor into K37 Gem 5. The Beast's mind then tried to attack, but Rose was able to shoot him into space and into K37 Gem 5. The Expedition's ship was also dragged into K37 Gem 5, but the Doctor was able to save them with his TARDIS. (DW: The Impossible Planet/The Satan Pit)
Behind the scenes
The black hole special effect was re-used in Invasion of the Bane, with the addition of some tumbling meteorites or asteroids.