Many thanks to SWLing Post contributor, John (KC8RZM), who writes:
[The following] is a fascinating little documentary I found on YouTube.
A secret bunker hidden inside Gibralter’s rock with a self-contained transmitter and self-generating power source (a bicycle frame).
The radio details start at the 11 minute mark:
Just thought some SWL might find it interesting, I sure did.
A fascinating documentary! Thank you for sharing, John!