Barbara Bain preferred working within a studio as opposed to location filming, enjoying "a world you were creating yourself", so she and Martin Landau refused to film away from the studio (even though Black Park - where the rest of the cast filmed without them - was adjacent to the studio property), so their outdoor scenes were filmed both on stage (outside the caves) and on Pinewood Studio's grounds.
The cast was looking forward to finally getting out of the studio for the filming of an episode (with the exception of Martin Landau and Barbara Bain). In an interview, Barry Morse recalled that during the shoot, they had to deal with periods of almost incessant rain, causing them to look forward to getting back into the studio.
This was the only season one episode filmed outdoors (with the exception of the rain scene on "The Last Sunset", which was shot out in the studio car park). Black Park, where most of this episode's outdoor scenes were filmed, was also used on "The Rules of Luton" and "A Matter of Balance".
Preparing for the scene where Sandra repeatedly bashes a rock into Commander Koenig's head, Barbara Bain asked Zienia Merton to take it easy on him. Zienia thought this was ironic, as in a previous episode "Guardian of Piri", Koenig had approached Sandra on the planet's surface, who was just standing there in a trance and to get her attention, he took her head in his hands, jerking it around to face him. Zienia was surprised she didn't end up in traction, and here Bain is worried that she'd hurt her husband with a Styrofoam rock.
This episode was filmed immediately after Death's Other Dominion (1975), so the ice caves used for Ultimate Thule were painted over for the Cro-Magnon caves on Retha.