In 2020, Tony Mack and Dr Nasreen Chaudhry are in charge of a drilling project in the tiny Welsh village of Cwmtaff. Their goal is to identify the source of unusual trace minerals in the local grass. But interred coffins have begun to vanish from the town cemetery, and now Tony's son-in-law has disappeared down a hole which suddenly opened in the ground. When the TARDIS arrives, Amy soon vanishes into the Earth as well. The Doctor and Rory lay a trap and snare a Silurian warrior called Alaya. They learn that a city full of her people has been reawakened by the drilling... and now they're ready to wage war against humanity.

By late 2008, Chris Chibnall believed that he had largely left Doctor Who behind him. His lone script for the series, 42, had aired as part of Season Twenty-Nine in 2007, and he had recently stepped down as co-producer and head writer of the spin-off series Torchwood at the conclusion of its second season. In October, then, he was surprised to find himself approached by Steven Moffat, the new executive producer of Doctor Who. Moffat had liked Chibnall's latter Torchwood scripts -- such as Adrift, Fragments and Exit Wounds -- and wanted to recruit him for Season Thirty-One. Moffat also knew that Chibnall was a lifelong Doctor Who fan, and so he had a project in mind that he felt would be particularly suitable.

Since its return in 2005, Doctor Who had gradually reintroduced many of the most memorable villains and monsters of its twentieth-century run: the Daleks, the Cybermen, the Master, the Sontarans and, most recently, Davros. For Season Thirty-One, Moffat wanted to bring back the Silurians, a race of lizard people who had been Earth's original dominant species until their belief in an imminent global catastrophe sent them into aeons of hibernation deep beneath the planet's surface. The Silurians were created by Malcolm Hulke for 1970's The Silurians, in which they encountered Jon Pertwee's Third Doctor. Two years later, Hulke gave them aquatic cousins in The Sea Devils. Then, in 1984, both species of Homo reptilia menaced humanity in Johnny Byrne's Warriors Of The Deep.

Chris Chibnall reacquainted himself with Malcolm Hulke's 1974 novelisation of The Silurians

Although the design of the Silurians had been broadly consistent across their previous appearances, Moffat encouraged Chibnall to devise a new branch of the species, which could be realised using make-up and costuming techniques unavailable in the Seventies and Eighties. In particular, Moffat wanted to dispense with the Silurians' third eye, which he felt was now more closely associated with Davros. Chibnall instead conceived the whiplike tongue with which the new Silurians could poison their enemies. Moffat also suggested that Chibnall reacquaint himself with Doctor Who and The Cave Monsters, Hulke's 1974 novelisation of The Silurians for Target Books. It took great pains to portray the Silurians as individuals, rather than as a homogeneous race, an approach which Chibnall found inspirational. To help convey the distinct personalities of the different Silurians, Chibnall recommended prosthetics which would allow much of the performer's facial features to remain visible. He also wanted to avoid any form of vocal modulation, as had previously been used for Silurian dialogue.

Chibnall was assigned a two-part slot for Season Thirty-One -- comprising episodes eight and nine -- and began writing scripts entitled “The Ground Beneath Their Feet” and Cold Blood. He wanted to give each installment a distinctive feel by changing the principal setting: after focussing on the human community of Cwmtaff in Episode One, the underground Silurian city would predominate in Episode Two. An influence was the Kola Superdeep Borehole, a Russian scientific project begun in 1970 on the Kola Peninsula near the Norwegian border. It eventually reached a depth of more than twelve kilometres in 1989, before falling victim to budgetary shortfalls after the demise of the Soviet Union; the drilling permanently ceased in 1995, and the project as a whole was abandoned in 2008.

Brief consideration was given to including a role for the Sea Devils. It was quickly agreed that their involvement would be an unnecessary complication, but the design of the Silurian weapons would echo those seen in The Sea Devils. Instead, Chibnall wanted to introduce a new monster called an Armasaur, which would function as a mobile weapon for the Silurians. Originally, it was these armadillo-like dinosaurs who were responsible for the abductions, and the holes in the ground were their routes to the surface. One reason for their inclusion was to allow the Silurians to remain unseen until the end of “The Ground Beneath Their Feet”, much like Hulke had delayed the first unimpeded sight of a Silurian until their original serial had reached the climax of its third installment. However, it soon became clear that the Doctor Who budget would not accommodate both computer animation for the Armasaurs and the sophisticated prosthetics necessary for the Silurians. As a result, the Armasaurs were dropped and the humans would instead be snared by bioprogrammed quicksand. To provide the episode with a more tangible threat, Alaya's introduction would now occur earlier in the narrative.

Tony Mack originally had a brother, and it was this character who murdered Alaya

Other elements dropped from the adventure included the Silurians animating tree roots in the graveyard to drag people into the ground. This was intended to be the mechanism via which the Doctor and Nasreen travelled to the Silurian city, before Chibnall realised that it would be simpler if they just used the TARDIS. Tony Mack originally had a brother, and it was this character who murdered Alaya. However, as he wrote his scripts, Chibnall came to feel that the strong feminine dynamic between Alaya and Ambrose Northover was more compelling; Ambrose became the Silurian's killer, and the brother was dropped. Likewise, Restac was initially conceived as a male Silurian.

In addition to the earlier Silurian stories, Chibnall echoed other classic imagery from the Pertwee era: the Discovery Drilling Project was reminiscent of the eponymous borehole experiment in 1970's Inferno, while the force field which encircled Cwmtaff was much like the heat shield around Devil's End in 1971's The Daemons. Two of the Silurian names were corruptions of Seventies Doctor Who personnel: Malohkeh for Malcolm Hulke himself, and Restac for script editor Terrance Dicks.

“The Ground Beneath Their Feet” and Cold Blood were made as Season Thirty-One's fourth recording block. The director was Ashley Way who, although a newcomer to Doctor Who itself, had previously worked on tie-in content, followed by six episodes of Torchwood. With Restac's change of gender, it was decided that she and Alaya would be played by the same actress: Neve McIntosh, who had starred in the fantasy adaptation Gormenghast and the medical drama Bodies. Meanwhile, Arthur Darvill was uncertain whether or not Rory's erasure from time at the conclusion of Cold Blood meant the end of his tenure on Doctor Who. He was booked on the show for several more months but -- because Chibnall's scripts were being made out of broadcast order -- he was yet to record his appearances in episodes six and seven, The Vampires Of Venice and Amy's Choice. Nonetheless, Darvill had heard rumours amongst the crew that Moffat was not finished with Rory Williams.

The production of “The Ground Beneath Their Feet” / Cold Blood began with a week in and around the twelfth-century St Gwynno's Church, in the Welsh hamlet of Llanwonno. Running from October 20th to 24th, the work there also took in the adjacent cemetery and the hillside where the TARDIS materialised. After a day off on Sunday the 25th, October 26th saw Bedwellty Pits in Tredegar serve as the area around the Northover residence. Tower Colliery in Hirwaun provided the exterior of the Discovery Drilling Project on the 27th, as well as its control room. On the other hand, scenes in the storeroom from which Mo and Amy were abducted were recorded at Mir Steel in Newport. Filming there took place from October 28th to 30th, with Danny Hargreaves of Real SFX taking advantage of an existing mechanics' pit to create the quicksand effect. Material in the drill site corridors was also taped on the last day at Mir Steel, after which the production stood down over the Hallowe'en weekend.

The next location for the Silurian story was Hensol Castle in Hensol, where sequences in the church crypt were filmed on November 2nd and 3rd. Alongside various inserts, clips of Amy and Rory enjoying each other's company were also captured on the second day; they would form part of Amy's futile attempt to remember Rory after his erasure from time. The remainder of the week -- from November 4th to 7th -- was confined to Doctor Who's regular studio home in Upper Boat. The first three days concentrated on scenes in the corridors of the Silurian city, with TARDIS material also taped on the 6th. On the last day, cameras rolled on the set for the chamber which housed the crack in time, with Rory's death amongst the sequences on Way's itinerary.

After the weekend, November 9th to 12th saw the Temple of Peace in Cardiff dressed as the Silurian Senate chambers. Way also captured some pick-up shots on the 11th, and remounted elements of Rory's final moments on the last day. The latter part of the 12th was spent back at Upper Boat, with the focus being the TARDIS scene which followed Rory's demise. On November 13th, Way's team travelled to the Plantasia botanical gardens in Swansea for footage of the Silurian city, such as the jungle walkway. Work there was followed by another full weekend off.

It was back to Upper Boat from November 16th to 18th. Most of the outstanding material was set in Malohkeh's laboratory, while Way also taped a variety of inserts and effects shots. Still required was a foot-angle perspective of the Doctor jumping up and down in the graveyard, which was taped at Upper Boat on January 13th, 2010. Finally, long shots of Mo on his bicycle were captured at the Vicarage in Rhymney on January 29th.

Dropped in editing was a conversation about faith with Alaya, which foreshadowed Rory's fate

Both The Hungry Earth -- as the first episode was renamed -- and Cold Blood were significantly overlength. This remained true even after the latter was granted a fifty-minute timeslot, as opposed to the usual forty-five. As a result, a number of important elements were cut from both installments. In The Hungry Earth, much was originally made of the fact that the Discovery Drilling Project was under pressure from its financial backers to reach greater depths more quickly. Cold Blood lost Amy and Mo's discovery that the Silurians had kidnapped a menagerie of modern animals, including a dog owned by Tony Mack. Also dropped in editing was an extensive conversation about faith between Rory and Alaya, which would have foreshadowed Rory's fate at the end of the story.

The Hungry Earth aired on May 22nd at 6.15pm. Doctor Who's previous lead-in, Total Wipeout, had finished its regular run the week before, so Young Apprentice was brought forward from earlier in the evening to replace it on this occasion. Following the Doctor and Nasreen's arrival in the Silurian city, the BBC broadcast the season finale of Over The Rainbow, which Doctor Who had preceded in the transmission schedule all year. On May 29th, Cold Blood aired amidst a disrupted BBC One schedule. It was pushed to a 7.00pm start time by that afternoon's Challenge Cup rugby quarter final between Leeds and Wigan. A rugby-themed edition of the quiz show Weakest Link was broadcast before Doctor Who, and Cold Blood was followed by The National Lottery Saturday Night Draws and then the final of the 2010 Eurovision Song Contest. This marked the first time since 2006 that Doctor Who was not postponed to accommodate the inclusion of Eurovision in the schedule.

• Doctor Who Magazine Special Edition #27, 16th March 2011, “The Hungry Earth / Cold Blood” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #65, 2018, “Story 209: The Hungry Earth / Cold Blood”, edited by John Ainsworth, Hachette Partworks Ltd.