El Altar volcano (5319m), located in Sangay National Park, is an extinct volcano shaped like a horseshoe after an immense eruption 500 years ago blew it to pieces. Prior to the eruption, it was one of the tallest mountains in South America. Now it is a spectacular natural amphitheatre, open on one side and bedecked with glaciers and a green lake (the colour caused by the rock sediment in the water). Reminded by its shape and grandeur of a cathedral altar, the Spanish named it “El Altar”, its peaks resembling two nuns and four friars listening to a bishop around a church altar – the altar being the spectacular central hanging glacier. In fact there are nine peaks around the jagged rim, and the local Quichua name Kapak Urku means, appropriately, “magnificent mountain”. Its highest peak, the vertically-walled El Obispo (the Bishop), is one of the most technically-demanding climbs in the northern Andes. The glaciers of El Altar have melted rapidly due to climate change and are now far higher up the mountain, feeding water into the amazing green lake in the centre of the amphitheatre.