The Solage Calistoga resort, nestled in a canyon in California’s Napa Valley, is home to the 20,000-square-foot Spa Solage, which offers healing, relaxing, and soothing services. Since 2007, guests of the spa have enjoyed treatments that use natural ingredients, from a seasonal hydrating green-tea massage and body wrap to the Mudslide, a three-part experience that involves having your body slathered with a custom blend of mineral-enriched clay and volcanic ash, soaking in a bath of mineral water from the …