These secret tunnels, used during the evacuation of Dunkirk in 1940 and the Battle of Britain, can now be explored on a 40-minute guided tour. The tunnels were originally excavated to house cannons to be used (if necessary) against an invasion by Napoleon. Some 60m (200 ft.) below ground, they were the headquarters of Operation Dynamo, when more than 300,000 troops from Dunkirk were evacuated. On once forbidden ground to all but those with the strongest security clearance, you can stand in the very room where Ramsey issued orders; experience the trauma of life in an underground operating theater; and look out over the English Channel from the hidden, cliff-top balcony, just as Churchill did during the Battle of Britain.