What was it?

The M Room was so secret that only the secret listeners who operated it and some intelligence officers knew of its existence.

The letter M stood for 'miked' and reflected the fact that the room was set up with the latest listening technology. Access was gained through two locked doors and the keys given only to designated staff. From here the operators could listen into the conversations of the prisoners in their cells or in one of the interrogation rooms. Sometimes the interrogations were recorded if prisoners started to give away important military information.

The monitoring of prisoners' conversations continued every day of the year, including Christmas Day, so that nothing was missed.