Scotland Yard firearms officers are using a disused Cold War nuclear bunker to prepare for rescuing hostages from armed criminals.
Teams of marksmen from the CO19 branch have begun staging a series of mock operations at the former radar station near Sandwich, Kent.
The underground building contains a labyrinth of rooms constructed after the Second World War set in 18 acres of secure grounds.
It now hosts a company offering secure data storage for banks, technology companies and local authorities.
Staff at the base have been posing as hostages in scenarios which focus on rescuing people without injury or damaging sensitive computer equipment.
Ray Welsh, of The Bunker, which owns the site, said the training has been taking place in unoccupied parts of the building.
He said: "It certainly livens up the day a bit but they have not caused us a large amount of disruption.
A Met spokesman said: "Using a variety of locations and premises also helps us evaluate the effectiveness of our response to incidents."