Flat head Lake is a very large freshwater lake located in the north western corner of Montana. With a surface area of roughly 190 square miles, the lake is actually a remnant of a massive inland sea. The site is an ancient, massive glacial dammed lake, from the last inter glacial period, which covered much of the region around 13,000 years ago. The lake is relatively deep, with a maximum depth of 370 feet. It is known for its stunning crystal clear waters, which give the illusion of much shallow waters. However this really is an illusion and you have a beautifully clear view to the very bottom of the lake.