Even if you’ve exhausted every last one of London’s above-ground activities, there’s still plenty of mystery, and history, underfoot. On the outskirts of Greater London in a storybook hamlet, the Chislehurst Caves offer a subterranean family day like no other. A demure yellow brick entrance building belies 22 miles of man-made tunnels, reaching back through generations of Druids, Romans, Saxons, and modern Britons, each with their own historical section of pitch. After you’ve secured your antique oil lamp and assured your kids that there are no dragons in this labyrinth, your guide will escort you down a steadily declining, increasingly dim walkway into the cave entrance. Since you’re essentially traveling back in time, your first stop is also one of the most recent, a World War II air raid shelter, where you can almost hear the murmurs of the 15,000 Londoners it protected during the Blitz. After inspecting the stone chapel, you’ll continue through the Saxon portion of the tunnels and then on to the mysterious Druid section near the back. Whether the small platform under modern artist Sandy Brown’s intricate stone carvings was a sacrificial altar, or a simple platform for mining chalk and flint, is still up for debate, but macabre mysteries like these simultaneously delight mom and dad while giving the little ones the heebie-jeebies. By the time you’ve returned from the structured Roman portion of the mine–where London’s very foundations were quarried–your kids will be casting an eye into every corner, anxious to catch the next small detail from centuries past.