The spring at Altstraßbach was captured in the 1940s during construction work for the Westwall.

 It is moderately mineralised, but has a high CO2 content of 3500 mg per litre. The water temperature varies between 6°C and 11°C. This is a "mofette" spring. The water circulating in the loosening zone and some fissures has only a short residence time and is forced upwards in the source area.

The direct access to the source bunker is closed by a door for safety reasons. With little air movement, an invisible CO2 layer up to 20 cm high collects at the bottom of the bunker. Animals and humans who come too close to this area are in mortal danger if they inhale the gas! Particular care is needed with dogs and fallen children. Only a few breaths are enough to die. Visitors should also do the CO2 test outside the walls with a candle or lighter: if the flame goes out, the CO2 concentration is very high.

Coordinates: 50°16.177'N, 6°44.452'E
Community/Place: Walsdorf-Zilsdorf
Altitude: 557 m above sea level
Water temp.: 6.0°C to 11°C

Approach: B421 towards Zilsdorf, in Zilsdorf turn onto Talstraße, the spring is on the left opposite the wind turbines.