Anyone who travels frequently in Vienna cannot avoid it. When changing trains at Stephansplatz subway station, you notice a strangely unpleasant smell, especially on hot days. Many urban legends and hidden secrets have circulated around this square – such as the ghost station of the U2 underground line or the medieval crypt directly beneath the cathedral. But the stench in the subway station still remains a mystery to many. From the remains of old horses and damage from earlier times to hygiene myths, there are many old stories. But there is actually a very specific chemical phenomenon behind the smell.
Not an urban fairy tale: The origin of the smell
When Stephansplatz station was built in the 1970s, engineers used a special organic-based soil stabilizer. It was intended to prevent the subsoil from giving way and causing St. Stephen’s Cathedral to sink. What nobody suspected at the time was that the chemical led to a by-product that still persists today: the formation of butyric acid.
Butyric acid smells extremely unpleasant, reminiscent of rancid butter or vomit. The acid dissolves in warm weather and enters the tunnels and shafts with the groundwater. This is exactly what causes the penetrating smell that literally “gets up the nose” of many people, especially in the U1 area.
Why does the smell persist?
Wiener Linien regularly cleans and flushes the tunnels. And yet the smell cannot be permanently eliminated, as parts of the drainage systems remain behind cladding and cannot be completely cleaned. Complete removal of the source is therefore practically impossible. The cause will persist until the substances are completely degraded at some point, which could take many years.
However, we do not need to worry about health issues. The concentrations of butyric acid released are so low that they are harmless to humans. No long-term negative effects on the fabric of the building have been reported either, as it is mainly the smell – not the structural integrity – that is the problem. Stephansplatz will therefore not collapse at some point.
The stench in the Stephansplatz subway station is actually a modern “legacy” of the construction work: pure chemistry, no dark secrets. At least we don’t have to worry about harmful health effects. The phenomenon will probably remain with us for many years to come – and the walk to the U1 will continue to be done in a goose step…