Keri Crater Lake The legendary Big Blue Bowl, located in the western volcanic area of Iceland, is one of the seven largest crater lakes in the region. It was formed about 6500 years ago and has a water depth of 7 to 14 meters. Most of the crater lakes are surrounded by steep rock walls, and this crater lake has a relatively flat rock wall that can follow the rock wall to the lake. This is one of the few natural landscapes in Iceland that requires a fee and costs 400 kr. At this time, one of the seven largest crater lakes in Iceland was formed 6500 years ago. Compared with the crater lake we saw later, this crater lake is like a sleeping girl, and the lake is clear and blue. In fact, there is a problem I am puzzled. Why does the water in the Crater Lake not deteriorate for so many years?