The Wachau Valley is a 23-mile visual feast of tidy medieval villages, hilltop castle ruins and dense conifer forests, all set amid dramatically steep vineyards whose terraced slopes cling to the riverbanks of the winding Danube. Despite more than a thousand years as a winemaking center, the Wachau has only seen its wines burst into the ranks of the world's great whites in the past 20 years, with aromatic Rieslings and food-friendly Grüner Veltliners. Stateside, Austrian wine has become a tiny but fast-growing niche, with wine lovers, merchants and sommeliers increasingly turning to Wachau whites. Wine Spectator's Robert Camuto explains.