The massive eruption & collapse of Mt Mazama (8000 yrs ago) created the amazing caldera that is Crater Lake. Because the water is only refilled by precipitation (mainly snow) the water is very pure – and very deep with an avg depth of 1,148 ft with max depth of 1,943 ft. The blue of the water is almost indescribable. The cliffs around the lake also protect it – there is actually only one (very steep) trail down to the water.