South of Quito the Andes rise sharply on either side of the Pan-American Highway, creating a narrow corridor of fertile valleys that, along with Quito, are home to nearly half of Ecuador's population. Along this 175-km (109-mi) stretch between Quito and Riobamba are most of Ecuador's tallest volcanoes, including one of the tallest active volcanoes in the world, Cotopaxi. Alexander von Humboldt, the German scientist who explored the area in 1802, was so impressed by the landscape that he coined a sobriquet still used today: the Avenue of the Volcanoes.