Thermal chamber
The thermal chambers were a system of caves beneath the surface of Kastria utilised as living space by the Kastrians. There, they were sheltered from solar winds.
The race bank was kept in the cave network, as was the regenerator chamber, on floor three zero six. The chambers could be accessed by a lift from a dome above.
Eldrad's development of the spatial barriers allowed the Kastrians to live on the surface but, when he destroyed them, he forced his people back underground. Rather than face what King Rokon described as "a miserable subterranean existence", the Kastrian people "chose final oblivion".
One hundred and fifty million years later, Eldrad returned to Kastria in a female form and went into the thermal chambers. After triggering a booby trap, left in anticipation of his return, and becoming a victim of a specialised acid, Eldrad had the Fourth Doctor and Sarah Jane Smith take her to the regenerator chamber. They jointly supported Eldrad on their way, with the Doctor carrying her over an abyss.
Once he was revitalised and returned to his "true" male form, he searched the chambers for Rokon and found his body. Finding the race bank empty, the Kastrians having destroyed its contents, Eldrad decided to return to Earth and rule over the human race. He pursued the Doctor and Sarah through the thermal chambers. They positioned themselves either side of the abyss and used the Doctor's scarf to trip Eldrad, who plummeted into the depths. (TV: The Hand of Fear)
