10 Deadliest Alien Prisons In Doctor Who

Also known as 'The Bitter Pill', Krop Tor was an impossible asteroid-like planet orbiting a black hole. Suspended beneath the planet's surface: an ancient prison for the demonic creature known as 'The Beast'.

Appearing in 2006's The Impossible Planet/The Satan Pit, Krop Tor was an intimidating locale prone to shuddering earthquakes. Discovered by a research team, the archaeologists had no idea they'd find a demon buried miles below the barren world.

Ten miles down, burrowed into the planet's craggy rock face: the ruins of an ancient civilisation who had vowed to keep the Devil at bay. Beneath a trap-door: a seemingly bottomless 'Satan' pit.

TARDIS lost to an earthquake, the Tenth Doctor decided to scale the pit. There, he uncovered the Beast - a demon from before the universe's creation. Trapped on the planet, the technology used to hold the Beast was also keeping Krop Tor from falling into the black hole. However, any attempt to free the creature would also switch off the planet's gravity, causing The Bitter Pill to be consumed by the void.

In an attempt to save itself, the Beast possessed the humans' Ood servants, as well as archaeologist Toby Zed, in an attempt to escape confinement. When the human explorers managed to evacuate the crumbling world, the Doctor 'freed' the Beast and the planet began to drift into the black hole.

The possessed Toby Zed was then shot into space by Rose Tyler, and the Doctor escaped in his TARDIS which had crash-landed at the mouth of the prison. The two reunited as the Beast, and Krop Tor, were destroyed.

Intergalactic Travel Advice: Take a leap of faith.