"The worst European hotspot for PFAS" is located in Belgium, in the small town of Zwijndrecht, near Antwerp. There, for 25 years, the American factory 3M manufactured perfluorooctane sulfonate (PFOS), one of the oldest of substances that is highly toxic to health and the environment. In Europe, there are an estimated 2,300 other such "hotspots" and a total of 23,000 polluted sites. A recent collaborative survey estimates the cost of decontamination at 100 billion euros per year. How is this decontamination carried out in practice? And what is the cost for residents already affected by one of the biggest environmental scandals? Alix Le Bourdon, Anna Joris and Dave Keating report.