Hindu Kush (hĭnˈdō kŏsh) [key], a high mountain system, extending c.500 mi (800 km) W from the Pamir Knot, N Pakistan, into NE Afghanistan; rising to 25,236 ft (7,692 m) in Tirich Mir, on the border of Afghanistan and Pakistan. Glaciated and receiving heavy snowfall, the mountains have permanently snow-covered peaks and little vegetation. Meltwater feeds the headstreams of the Amu Darya and the Indus rivers. The irrigated valleys are heavily populated, and extensive lumbering has greatly reduced the forests on the southern slopes. The system is crossed by several high-altitude passes; once used by Alexander the Great, Timur, and others in their invasions of India, they are now trade routes. The Hindu Kush were called the Caucasus Indicus by the ancient Greeks.