Boulder, Colo., landscape architect Martin Mosko nestled this koi pond into the side and bottom of a Brentwood, Calif., canyon. The project's central attraction is a redwood tea house built upon a manmade peninsula. “The tea house is almost as much of a visual thing,” says Mosko, who carefully sited it to be visible from the main house, 25 feet up the hill. As the koi glide silently under clusters of lily pads, the water they swim through is clean enough to drink, according to Mosko. The multilayered pond contains a natural filtration barrier of volcanic rock; as water falls from the upper levels to the lower ones, the bacteria living in the rock digest particles of fish waste and other toxins. Aquatic plants provide further biological purification. “The idea is that the whole mountain breathes water,” Mosko says.