London - Locals had long suspected its existence, of course, following decades of spurious equinoctial and solstice Druid activity next door at Primrose Hill.
This morning's discovery of a huge subterranean gambling complex came as more than a month's rain fell on London causing widespread flooding and the odd landslip.
Keepers said they first spotted rivulets of gaming chips and IOUs signed by self-styled druid king Arthur Pendragon floating past the Diana Monkey House at the regular 6am early morning cage muck-out.
The flow then swelled to a torrent prompting an 8.30am 999 call to the London Fire Brigade amid reports of strange commotion under the nearby 1934 Lubetkin Penguin Pool.
Officially closed since 2003 the site had previously camouflaged the existence of a subterranean War Orifice bunker designed to protect the Cabinet and members of the royal family.
Erected during World War II in the widespread belief that the Luftwaffe 'would never bomb helpless dumb animals' the existence of the underground facility had retained its Top Secret classification.
This morning's emergency bale-out saw firemen smash down the gambling site's secret underground access points, installed and covered up at vast taxpayer expense under the guise of zoological conservation projects.
Reports that one of the connecting tunnels led the St John's Wood Blue Watch 'directly to Peter Mandelson's house' in nearby Park Village were hastily denied as firefighters toiled to bale out the waterlogged complex.
Last weekend the UK media was up in arms at news of the Prince of Darkness' alleged purchase of an £8million mansion half a mile away - with much spirited debate about the actual source of his new found wealth.
"There's no evidence of Mandy being a Druid," a Labour Party source said at lunchtime, "but he does enjoy dressing up as a joke and might go out in fancy dress on the odd occasion."
The next local Druid ceremony is scheduled for 23 September in commemoration of the annual Autumn Equinox.