Vinho Verde, Portugal’s largest wine re- gion, stretches from the Spanish border to south of Oporto. The soils are acid-rich and granite-based, with ar- eas of schist or slate and clay. The slopes tend to be gentle in Vinho Verde, though some vineyards are planted on steep or terraced slopes. The re- gion gets 1,500 mm (59 inches) of annual rainfall, which makes vinegrowing a challenge. Owing to this, many unorthodox growing techniques can be seen in the region, including vines encircling vertical poles. The trademark fizz of Vinho Verde wines derives from the release of carbon dioxide during malolactic fermentation.