When BT took over some of the GPO’s shallower ‘Chambers Deep’ they signed an extensive and detailed undertaking not to reveal the ongoing existence of ‘The O’ (as it’s known) or to pinpoint the location of any chambers. Unfortunately, bureaucracy being what it is, that message doesn’t always get to the on-the-ground engineers who have to know where the chambers are when they’re drilling or pulling glass. BT’s security officers are caught in the old dilemma - they can’t tell people not to talk about things they’re not allowed to talk about.

When, as here, a chamber is accidentally revealed it’s immediately logged by infrastructure exploration groups like X-Deep and dozens of ‘O-fficionados’ (amateur documenters of the GPO’s ongoing reality) will release trained mice, rats and coypu in the area - trying to sniff out entries to the chamber. They’ve not found one yet, but it’s only a matter of time.