The TARDIS brings the Doctor and Rose to an Earth research base on a planet which, against all the laws of physics, is orbiting a black hole. The crew are drilling into the surface, trying to unearth the power source counteracting the black hole's massive gravitational pull. But Toby Zed, the archaeologist trying to decipher ancient runes found on the planet, is being haunted by a malevolent voice. And the Ood, the servitor race staffing the base, seem to be falling under the sway of an evil from the dawn of time...

A common criticism levelled at the first season of the new Doctor Who series had concerned its lack of alien environments, with all thirteen episodes taking place either on Earth or aboard a space station in Earth orbit. This had been intentional on the part of executive producer Russell T Davies; apart from the cost involved, he felt that every Doctor Who story needed a human connection to seem relevant to the viewer, and he also feared that audiences would not buy into other planets as plausible settings. For the 2006 season, however, Davies was confident enough to attempt an adventure partly set on another world. He drew on the 1987 John Carpenter film Prince Of Darkness in conceiving a plot about explorers becoming possessed by demonic forces, which he also wanted to serve as a challenge to the Doctor's own belief systems. Davies was also responding to producer Phil Collinson's observation that none of the Doctor's enemies during the first season had been godlike in scale.

The writer assigned to this storyline in February 2005 was Matthew Jones. Jones was a longtime fan who had penned the 1996 Doctor Who: The New Adventures novel Bad Therapy for Virgin Publishing. He also enjoyed a busy career in television, earning writing credits on Coronation Street and Love In The 21st Century. As a script editor, Jones had tackled shows such as the Davies-created Queer As Folk and had latterly become a producer for programmes including POW (which he also created) and Shameless. As originally envisioned, the planet in Jones' story -- given the overall title “Satan Pit” -- was actually called Hell. Jones also intended to hold back the revelation of Toby's continued possession in part two until the climax; this was altered for fear that it was too abrupt.

A more substantial change occurred when Jones determined that “Satan Pit” would benefit from the presence of a monster throughout the story. Jones asked Davies if he could use the Slitheen -- or, more specifically, another family from the planet Raxacoricofallapatorius -- with the idea being that, by the time of “Satan Pit”, they had become enslaved by humanity. The Raxacoricofallapatorians secretly believed that the planet being explored was actually the dwelling place of their god, who would free them from servitude. However, Davies ultimately became concerned that the presence of the returning monsters was detracting from the humans whose pioneer spirit he felt was at the heart of the tale. He was eventually able to budget for the creation of a new race he christened the Ood (whose simple name was an intentional contrast to Davies' typical flair for ostentatious alien monikers).

Also of great concern was the form which the Beast would take. Although the two parts of “Satan Pit” were intended to be the seventh and eighth episodes of the season, they would be made towards the end of the schedule, when money was typically starting to run low. Fearing that the production would not be able to afford an elaborate CGI creation, Davies came up with a number of alternative suggestions which he offered to Jones, such as an old man or a gigantic eye, and was especially fond of having the Beast appear in the form of a young girl (inspired by one of the avatars of the Senior Partners in the vampire drama Angel). Davies also briefly considered using the future-human concept he had originally offered for use in Dalek when it appeared for a time that the rights to use the Daleks themselves would be unavailable. In the end, however, money was eventually reallocated to allow the computer-animated realisation of the Beast.

The two episodes of “Satan Pit” were accorded the titles The Impossible Planet and The Satan Pit. These were intended to be made as the fourth recording block of the season, directed by Euros Lyn. However, with development still ongoing -- particularly to replace the Raxacoricofallapatorians with the Ood -- it was decided to hold Jones' scripts back, and so The Idiot's Lantern and Fear Her were assigned to Block Four instead. By this point, it had also been determined that the two installments would air eighth and ninth in the running order, as The Idiot's Lantern had been brought forward to avoid having two multi-part stories (the other being Rise Of The Cybermen / The Age Of Steel) broadcast consecutively.

The Impossible Planet / The Satan Pit now comprised the fifth production block, under the aegis of James Strong. After starting out in television as an actor, presenter, and reporter, Strong had moved into directing, earning credits on Holby City, The Afternoon Play and Casualty, amongst others. He also began making short films. Block Five would be filming in parallel with the year's sixth and final recording block, which included only Love & Monsters. This was a time-saving scheduling decision which also helped reduce the strain on David Tennant and Billie Piper; such double-banking had also been carried out in 2005 when The Long Game entered production while filming was still ongoing for The Empty Child / The Doctor Dances.

Recording began on February 28th, 2006 at Wenvoe Quarry in Wenvoe, which served as the cavern containing the seal on the Beast's prison. Although filming in quarries had been a staple of the original Doctor Who series, this was the first use of such a location in its 21st-century iteration. Since an open quarry was standing in for an enclosed cave, Jones had written extra dialogue to cover the possibility of rain or snow, with the Doctor explaining either as poisonous precipitation produced by gases circulating beneath the planet's surface. Shooting at Wenvoe Quarry continued through March 3rd; the same day, some footage of the Doctor descending into the pit was completed at HTV Studios in Culverhouse Cross, Cardiff.

Four days at the regular Doctor Who studio space of Unit Q2 in Newport followed, spanning March 6th to 9th, for scenes on the Sanctuary Base walkways and access tunnels. On the 10th, Tennant travelled with the crew to Clearwell Caves, near Coleford in Gloucestershire, where he filmed the material in the Beast's prison. The next day was again spent at Wenvoe Quarry, which now stood in for the planet's surface in addition to its interior. Scenes in Toby's room and the rocket were filmed on March 13th and 14th at the Enfys Television Studios in Cardiff. The next two days shifted back to Unit Q2 for recording on the habitation area set, before returning to Enfys on the 17th where Strong concentrated on segments involving the capsule and the pit interior.

The Johnsey Estates factory in Pontypool played host to the production for six days beginning on March 20th; it posed as both the Ood holding pen and the bore room. Various shots were then completed at Unit Q2 from the 27th to the 31st, most notably those in the command centre and the TARDIS console room. With recording on Love & Monsters having started and finished while The Impossible Planet / The Satan Pit was in production, March 31st marked Billie Piper's final day of filming on Doctor Who. Piper would immediately be starring in a BBC production of the Philip Pullman novel The Ruby In The Smoke; her exit from Doctor Who was confirmed to the press on June 15th. Apart from the TARDISode for The Satan Pit, this was also the last work performed for the series at Unit Q2. A new purpose-built dedicated studio space had been constructed for Doctor Who at Upper Boat near Cardiff, and the programme's only standing set -- the TARDIS -- would be relocated there during the spring.

Although The Impossible Planet / The Satan Pit was now largely complete, three further days of filming were still necessary. The most critical of these took place at the fabled Pinewood Studios in Iver Heath, Buckinghamshire, on April 1st. There, a water tank was employed for the shots of Scooti and Toby floating through space. Three days later, the smoke and flame elements needed for special effects shots were recorded at the Ealing Television Film Studios in London. Finally, a pick-up shot of Rose's hand unlatching Toby's belt buckle was taped at Enfys on April 11th. The only significant edit to the two episodes in postproduction was a sequence in which the Doctor uses the drillhead to detect the sound of a heartbeat emanating from beneath the planet's surface.

Writer Gareth Roberts' TARDISode for The Impossible Planet depicted Captain Walker (who has died by the time the episode itself starts) being assigned the expedition. The 58-second item ended with Walker being watched by an Ood, who utters a prophecy of the Beast. Roberts' original conception of the piece was slightly different, being set in a docking station prior to the start of the mission rather than an executive office. Recording took place at BBC Broadcasting House in Cardiff. The TARDISode for The Satan Pit was a sequel of sorts to the previous prelude. Using the command centre set at Unit Q2, it depicted a Sanctuary Base crewmember discovering documents relating to the legend of the Beast amongst the now-deceased Walker's personal effects, only to be victimised by the strange phenomena associated with the Beast's awakening. The 57-second item conclued with another crewmember finding the man's corpse, covered in symbols. This was intended to tie into references in The Impossible Planet to people having died in the command centre, but unfortunately the relevant dialogue was excised in editing. Both TARDISodes were taped on April 1st.

• Outpost Gallifrey: New Doctor Who Series News Page, edited by Shaun Lyon.
• Doctor Who Magazine Special Edition #14, 9th November 2006, “Episodes 8 & 9: The Impossible Planet / The Satan Pit” by Andrew Pixley, Panini Publishing Ltd.