“During the winter,” to quote the ever reliable Wikipedia, “the lake is only 1–2 m deep and the surrounding area is used as a county park. However, during the spring, when the temperature rises and the water melts, the basin of land below the mountains fill with water. The lake reaches its maximum depth of around 12 m from mid-May to June.” Seeing all that amazingly crystal-clear water and that submerged meadow freshly efflorescent, there's no wonder why it's a popular diving spot.
While that's hardly in an urban setting, I am nevertheless reminded of a festival in Rome once held during the city's sweltering summer. It's one my ultimate favorite spatial stories, and it involved blocking the drains of the Fontana dei Quattro Fiumi at Piazza Navona and letting the waters overflow and create a mini-lake. Crowded with all manner of vehicles, it was a sort of reincarnation of antique naumachiae, or mock naval battles, that may have been staged on that very same piazza, inside the former Stadium of Domitian, more than a thousand years previously.
This is how the poet Henry Wadsworth Longfellow described this urban cryo-spectacle:
Every Saturday afternoon in the sultry month of August, this spacious square is converted into a lake, by stopping the conduit-pipes which carry off the water of the fountains. Vehicles of every description, axle-deep, drive to and fro across the mimic lake; a dense crowd gathers around its margin, and a thousand tricks excite the loud laughter of the idle populace. Here is a fellow groping with a stick after his seafaring hat; there another splashing in the water in pursuit of a mischievous spaniel, who is swimming away with his show; while from a neighbouring balcony a noisy burst of military music fills the air, and gives fresh animation to the scene of mirth. This is one of the popular festivals of midsummer in Rome, and the merriest of them all.”
This temporary, theatrical reemergence of the marshy landscape on which the Eternal City was built, unfortunately, was last staged in 1866.
Setting aside concerns over peak-water (“It's the heat, I tell you!”), a city hollows out one of its parks into a crater (or an archipelago of parks, not just one; or maybe add quarries into its park system). When the temperatures are forecast to hit above 100°F for more than a day, the dams are uncorked. Along the shallow periphery, children frolic, while on deeper waters, scuba divers slither over and under park benches and swing sets, round public art installations in the round, and cool down next to drowned fountains. For the more adventurous, there are artificial cenotes filigreed with tunnels. Forget roof gardens and depaving asphalts, this is how cities should cool themselves.
Another city, gripped with delirium, decides to gouge its glacier-flattened grid with deep canyons, which are then plastered with meadows and planted with flowering orchards. Ur-dreams of fjords come true. At the start of its Midsummer Festival, the locks lining its Great Lake are raised, and this irrational exuberance in topography is transformed into a micro Marianas Trench, hosting mock sea battles, flotillas, pop-up aquariums and James Cameron.