The foul well, which is now located on Platz der Deutschen Einheit, takes its name from the smell of hydrogen sulphide in its water. This is caused by chemical reactions controlled by bacteria in layers of the subsoil containing lignite and sulphur gravel. The Faulbrunnen was always a municipal spring and belonged to the city of Wiesbaden. The mineralization of the water is significantly weaker than in the Wiesbaden thermal springs. As the water temperature only reaches a maximum of 17 °C, the water is not thermal water. In contrast to the other Wiesbaden mineral springs, at the Faulbrunnen the sericite gneiss characteristic of Wiesbaden is overlaid by sediments from the tertiary sea near the edge of the Mainz Basin.