Doctor Who: 15 Cool Details Peter Harness Just Revealed About The Zygon Invasion
9. The Tunnels From Flatline Were Reused Due To Budget Limitations
Twenty minutes into The Zygon Invasion, Clara rides a lift and finds herself in a massive, sparsely-lit tunnel, which leads to an area packed with Zygon pods.
This tunnel is the same one that was used in Flatline, an episode from Capaldi's first series as the Doctor. Harness' original idea was for the Zygons to inhabit abandoned Tube stations, but this notion was scrapped due to budget limitations.
Harness also stated that the idea of using a lift to reach the tunnel was thought up by script editor David Davis. Team effort!