Check out these brilliant illuminated cave pools at La Grotte de St Marcel d’Ardèche in France. According to the official website, the cave was discovered in 1836 and over 57 km of galleries have been explored and listed to date.
The pool formations are known as rimstone, or ‘gours’, and are made up of calcite and other minerals that build up in cave pools. Rimstone dams form where there is some gradient, and hence flow, over the edge of a pool. [source]