It was my second visit to Crater Lake, the blue jewel of Oregon. This round and very deep lake formed inside a massive crater left behind when a large volcano known as Mount Mazama collapsed about 7700 years ago. The water is completely sealed off inside this gigantic bowl of rock. There are no inlets or outlets. Only rain and snow contribute to Crater Lake, which therefore has some of the purest and clearest water in the world. This water clarity combined with its great depth (the deepest lake in the United States) causes the intense blue color. The sunlight can travel deep into the clear water, but only short-waved blue light is reflected back.