Have you ever gone swimming in a lake in Washington State and wondered how deep it was?. I grew up swimming and waterskiing on Lake Roosevelt, and I always knew it was deep. I had no idea however just how many deep lakes there are in Washington State. Some of the deepest lakes in the world are located here including one that is the 3rd deepest in the USA and the 28th deepest on our planet. This particular lake is over 50 miles long and was formed by a glacier cutting deep into mountainous rock.