Built on a toxic peninsula wasteland on the Thames in the 1990s, the Millennium Dome was conceived as a showplace for what turned out to be a poorly attended turn-of-the-century exposition. It’s supported by a dozen 100m-tall yellow towers, one for each hour on the clock, in honor of the nearby Greenwich Prime Meridian—a preening and meaningless symbolism that cost £789 million, and then stood empty for half a decade. In 2007, however, it was finally reborn as the city’s finest performance arena, with 20,000 seats. Now, Londoners can stomp on it like they always wanted. Climbers, about 10 at a time, hook into a safety rigging system and follow a guide over a tensile fabric catwalk laid a few feet over the Dome’s roof, to an observation platform at the zenith of the structure. There they pause for 15 minutes of photos of East London (the City is mostly hidden behind Canary Wharf’s towers). Beneath them, humming like a ship at sea, is a Dome conquered. The excursion isn’t for the height-averse—at your highest, you’re 52m (171 ft.) above the ground—and it’s not for big eaters or children, either (the weight cutoff is 130kg/286 lb., and you have to be at least 10 years old). But it’s also not scary, since you’re tethered and the shoes they lend you grip well. If the weather’s bad, you get matching jumpsuits like Ooompa-Loompas. The climb, which is more like a stroll up a steep hill, takes 45 minutes; the rest of the 90-min. experience consists of getting harnessed and psyched up.