A small lake formed by a carbon-dioxide-bearing rising spring issuing from Upper Jurassic carbonate rocks, which noisily "boils" with bubbles of CO₂. The spring yields 2.5 million liters a day; the lake's area is 0.04 ha. In windless weather the release of gas causes carbon dioxide to accumulate in the lower layers of air. Small animals approaching the lake suffocate, so the bodies of dead mice, lizards, frogs and birds may be found here.