A solfatar in the crater of the Mutnovsky Volcano. Solfatar is a kind of volcanic gas predominantly containing sulfur. This gas is easily recognized because the smell looks like a bad egg. Usually, around the hole of gas emission, yellow and acid sulfur sublimate is deposited. The other meaning of solfatar is a kind of fumarole emitting sulfurous gasses. Fumarole is an opening in Earth's crust, which emit steam and gases. From the perspective of groundwater, fumaroles could be described as a hot spring that boils off all its water before the water reaches the surface. Walking in the crater of an active volcano one have the imagination to being in the hell. The high temperature, smell of sulfur, wreathes of steam, fountains of boiling water, fire, streams of lava: all these create a unrealistic, threatening scenery. There is an unusual acid lake in the crater of a Maly Semyachek Volcano. The size of the lake amazes: it is about half kilometer wide and is 140 m deep. The temperature in this opaque, sometimes turquoise sometimes green lake, ranges from +27 °C to +42 °C while the level of mineralization is equal to an average concentration of sulphate and hydrochloric acids. A steel cane put in the lake gets "swallowed" in several hours.