The show’s official Twitter account has published this image, showing one of the shapeshifting aliens on what could well be a Welsh hillside.
The Zygons first appeared in the 1975 story Terror of the Zygons opposite Tom Baker’s Doctor and UNIT.
Although they only appeared on screen once, the Zygons became a big fan favourite and have appeared in a number of spin-off novels and comic stories.
Filming the untitled 50th Anniversary Special started this week. Over the weekend the BBC released images of David Tennant and Matt Smith at the script read-through.