Lake Sfânta Ana is the only crater lake in Romania located in the volcanic crater of the volcano named Ciomatu Mare of the Eastern Carpathians, near Tușnad in the Natural Reserve of Mohoș, Harghita County, Romania. It has an oval form and an area of 220,000 m².
The maximum depth of the lake is 6.4 m and the sediment thickness is about 4 m. The lake is supplied exclusively from precipitations, therefore the degree of mineralization of the water is very low. The water purity approaches that of distilled water, with only 0.0029 ml mineral.
In winter, the lake is covered with a layer of ice of up to 1 m.Near the lake, there is a Roman Catholic chapel dedicated to Sainte Ana.