Bali’s central mountains include several peaks over 2,000 metres (6,600 feet) in elevation and active volcanoes such as Mount Batur. The highest is Mount Agung (3,031 m, 9,944 ft), known as the “mother mountain”, which is an active volcano rated as one of the world’s most likely sites for a massive eruption within the next 100 years. As of late 2017 Mount Agung has started erupting and large numbers of people have been evacuated, the airport in Bali has been closed. The extent of the eruption is as of November 2017 impossible to predict. Mountains range from centre to the eastern side, with Mount Agung the easternmost peak. Bali’s volcanic nature has contributed to its exceptional fertility and its tall mountain ranges provide the high rainfall that supports the highly productive agriculture sector. South of the mountains is a broad, steadily descending area where most of Bali’s large rice crop is grown. The northern side of the mountains slopes more steeply to the sea and is the main coffee producing area of the island, along with rice, vegetables and cattle. The longest river, Ayung River, flows approximately 75 km (47 mi) (see List of rivers of Bali).