The only crater lake in all of Romania, Lacul Sfanta Ana is located inside of the volcanic crater named Puciosul of the Eastern Carpathians. It is nestled near Tusnad in the Natural Reserve of Mohos, Harghita County.

Palynology studies have concluded that this lake, Lake Saint Anne as it is known outside of Romania, was created somewhere between 9,800 and 8,800 years ago. Then, it was just an area of peat bog with a shallow lake in the center. Today, the lake measures more than 220,000 square meters even though the maximum depth is just 6.4 meters, with sediment thickness adding another four meters.

The size that Lake Saint Anne has grown to is remarkable, especially when you consider that it is supplied exclusively by rainwater and precipitation in the area. Because of this, though, the degree of mineralization of the water is very low; the water held inside of this crater lake is almost as pure as distilled water one can buy in a store. Also, no animal that can survive in the oxygen-filled waters of Lake Saint Anne has ever been transported in or found its way inside of the crater lake, leaving it entirely devoid of life.

Today, visitors to Lake Saint Anne know if they are going to have a clear day for hiking and picnicking based on two cracks that formed in the mountains long ago.

Because the mountain is still experiencing some post-volcanic activity, it is sensitive to any change in air pressure. When the pressure decreases, gases such as sulfur and carbon dioxide rise to the surface. If visitors they smell rotten eggs, it's clear that a storm is forming overhead.