Booby trap
A booby trap was a secreted device that would attack those who triggered it.
Knowing there was a chance Eldrad would return to Kastria, the Kastrians placed booby traps in their subterranean dwelling.
When Eldrad did come back, after one hundred and fifty million years, a door he opened triggered a booby trap. A dart containing an acid that neutralises the molecular bonds and eventually will shatter the crystal matrix in his silicon-based body hit him. As there was no antidote, the acid being of Eldrad's own design, he asked the Fourth Doctor to deliver him to a regenerator chamber.
They dodged other traps on the way, including a motion triggered gas in a passageway on the level 306 and some falling rocks.
The door of the chamber was also fitted with a booby trap, which the Doctor detonated with no ill effects. (TV: The Hand of Fear)
