This set was not added until mid-way through the run of the show.  
(Previously, the door to this chamber instead led to a turbolift).  The Jefferies 
Tubes themselves were ingeniously designed so that relatively short 
horizontal and vertical tube segments could be filmed in such a way that 
when a scene was cut together, it could look like our heroes were crawling 
through a lengthly, complex network of tunnels.  Æ