The TARDIS brings the Doctor, Amy and Rory to the tiny Welsh village of Cwmtaff in the year 2020. There, a drilling project seeks to burrow deep beneath the surface of the Earth. Strange craters have begun opening up near the drill site, however, dragging people into the ground -- and Amy becomes the latest victim. Investigating, the Doctor realises that the drill has awakened a tribe of Silurians from their aeons-long slumber. Believing themselves to be under attack, the Silurians are now on a war footing, preparing an offensive against the human race.

By late 2008, Chris Chibnall had largely left Doctor Who behind him. His lone script for the series, 42, had aired as part of Season Twenty-Nine, and he had recently stepped down as co-producer and head writer of Torchwood following its second season. In October, then, he was surprised to find himself contacted 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. In particular, Moffat knew that he was a lifelong, diehard Doctor Who fan, and so he had earmarked Chibnall for a special project.

Since its return in 2005, Doctor Who had gradually been reintroducing the most memorable villains and monsters of the classic era. To date, these had included 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 an anticipated disaster sent them into aeons of hibernation deep beneath the surface. The Silurians had been created by Malcolm Hulke for 1970's The Silurians in which they encountered Jon Pertwee's Third Doctor, and the writer had also given them aquatic cousins in The Sea Devils two years later. Both species of homo reptilia had then appeared in 1984's Warriors Of The Deep.

Although the appearance of the Silurians had been broadly consistent between their two previous serials, Moffat encouraged Chibnall to devise a new branch of the race for their twenty-first century revival. In particular, Moffat wanted to dispense with the Silurians' third eye, which he felt was now associated with Davros. In its place, Chibnall conceived the whiplike tongue with which the new Silurians could poison their enemies.

Chibnall was given a two-part slot for Season Thirty-One, and began writing episodes entitled “The Ground Beneath Their Feet” and Cold Blood. In addition to the earlier Silurian stories, the writer echoed classic imagery from the Pertwee era: the Discovery Drilling Project was reminiscent of the eponymous experiment in Season Seven's Inferno while the force field the Silurians created around Cwmtaff was much like the heat shield around Devil's End in the Season Eight finale The Daemons. Chibnall also suggested that the Silurian guns could bear a marked resemblance to those wielded by the Sea Devils.

One major story element that was ultimately excised from Chibnall's scripts was a new monster called the Armasaurs, armadillo-like dinosaurs which would emerge from the Silurians' shafts to abduct people. It soon became clear that the budget for the episodes would not permit both the Armasaurs and the sophisticated prosthetics necessary for the Silurians. Thus the Armasaurs were replaced by the bioprogrammed quicksand, and Alaya was introduced earlier in “The Ground Beneath Their Feet” to provide a mobile, sentient threat.

The Silurian story would be made as the fourth recording block for Season Thirty-One under director Ashley Way. Way was a newcomer to Doctor Who proper, but his work on the interactive episode Attack Of The Graske and the downloadable TARDISode prequels for Season Twenty-Eight had led to no fewer than six credits on Torchwood, including Captain Jack Harkness, Kiss Kiss, Bang Bang and Exit Wounds. Away from the world of Doctor Who, Way had also directed episodes of Casualty, Belonging and Crash.

The production of “The Ground Beneath Their Feet” and Cold Blood began in October 2009. Ironically, although Rory's death at the story's climax was a major turning point for the entire season, Arthur Darvill had played the character only once before, in the just-completed The Eleventh Hour. Darvill knew that he would be filming two additional episodes -- The Vampires Of Venice and Amy's Choice -- which would air prior to the Silurian story, but at this point he was uncertain whether or not this was truly the end of Rory.

The first five days of filming, from October 20th to 24th, took place in and around St Gwynno's Church in the Welsh hamlet of Llanwonno, including the adjacent cemetery. The area around the Northover residence was actually Bedwellty Pitts in Tredegar; Way's team was there on October 26th. The next day, Tower Colliery in Hirwaun provided the exterior of the Discovery Drilling Project, as well as the control room. However, scenes in the storeroom from which Mo and Amy were abducted were recorded at Mir Steel in Newport from the 28th to the 30th; material in the drill site corridors was also taped on the 30th.

November saw cast and crew travel to Hensol Castle in Hensol, where sequences in the church crypt were filmed on the 2nd and 3rd. Clips of Amy and Rory enjoying each other's company were also captured on the second day, to form part of Amy's vain attempt to remember Rory after he is erased from time. November 4th to 7th saw the first studio scenes recorded for “The Ground Beneath Their Feet” / Cold Blood at Upper Boat Studios; these chiefly took place in areas of the Silurian city, although the TARDIS set was in use on the 6th. Then it was back out on location from the 9th to the 12th, with the Temple of Peace in Cardiff dressed as the Senate chambers. Some elements of Rory's death were also recorded there, and this work continued back at Upper Boat on the 12th, when further TARDIS material was also completed.

More scenes in the Silurian city were filmed on November 13th at the Plantasia botanical gardens. This work continued at Upper Boat from the 16th to the 18th -- particularly for those sequences in Malohkeh's laboratory. Various insert shots were also completed at this time. While this marked the end of principal photography on “The Ground Beneath Their Feet” / Cold Blood, a foot-angle perspective of the Doctor jumping up and down in the graveyard was taped at Upper Boat on January 13th, 2010, and long shots of Mo on his bicycle were captured at The Vicarage in Rhymney on January 29th.

Both The Hungry Earth -- as the first episode was renamed -- and Cold Blood badly overran, even despite the latter being granted a fifty-minute timeslot as opposed to the usual forty-five. As a result, Way found himself cutting a number of important elements from both episodes. 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 -- a plot strand to which Meera Syal, who played Nasreen, made reference in some of her media interviews for the serial. Cold Blood lost Amy and Mo's discovery that the Silurians have kidnapped a menagerie of modern animals, including a dog owned years earlier 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.

• Doctor Who News.
• Doctor Who Magazine Special Edition #27, 16th March 2011, “The Hungry Earth / Cold Blood” by Andrew Pixley, Panini Publishing Ltd.