The Bottomless Lake (a.k.a. Francisc Lake or Wooden Lake) is one of the 52 salt lakes, which are located in the famous spa Ocna Sibiu. Situated in an area of hills, the lake was formed when the ceiling of the Francisc Grube (Francisc salt mine), closed in 1775, collapsed.
Currently, it has an oval shape, with diameters of 40 m and 50 m, and with an area ranging from 1384 sqm and 1655 sqm, a volume of 11,114 cubic meters and a truncated cone shape, with a maximum depth of 34,5 m. The salinity of the lake is 9 g/l at the surface, and grows very quickly with depth: 38 g/l at 1 m, 122 g/l at 1.5 m, 203 g/l at 2 m, 320 g/l at 5.5 m.
The importance of this lake is related to the chemical-thermal regime. Water temperature is depending on the vertical distribution of salt concentration, which increases in depth. Lake water accumulates solar heat during summer and retains a partially and in winter. This phenomenon is called "heliothermia" and manifests the strongest, reason for which the Bottomless Lake was declared protected area, Class IV, according to IUCN classification, "nature reserve", a natural monument with geological value.
To maintain heliothermia phenomenon is necessary that the surface water layer will not be shaken. Flora and fauna of the lake are directly influenced by climate characteristics and water chemistry, as the bacterium Beggiatoa.