ro.wikipedia.org/wiki/Lacul_Sf%C3%A2nta_Ana
The lake St. Ana, located in the crater of Ciomatul Mare, is the only volcanic lake in Central and Eastern Europe, being the most visited travel destination of the Szeklerland.
The steep walls of Ciomatul Mare’s volcanic cone surround it, with a few higher peaks: Ciomatul Mare (1301 m), Ciomatul Mic (1238 m), Tata hill (1174 m) or Pietros peak (1125 m). Unique in Europe, the lake is at 946 m altitude, with a diameter of 1737 m and a maximum depth of 7 m. From the edge of the crater, 13 temporary brooks, which form during heavy rains, bring water and gravel into the lake.
The lake is fed by rainwater and snow melt. The process of sedimentation began on the northern and western edges, an increasingly thick layer of sludge covering the bottom of the lake. When the weather is fine, especially without wind, you can see bubbles of gas rising from the bottom of the lake, which are actually manifestations of post-volcanic activity. In the north and northeast of the lake, one can see a layer of floating peat about one meter thick.