By scouring the sewers of European urban areas, scientists have managed to take a peek into the hidden drug habits of 50 different cities across the continent.

The recent study by the Sewage Analysis CORe group Europe (SCORE) in association with the EU drugs agency EMCDDA analyzed daily samples from wastewater treatment plants in over 50 cities in 18 different European countries over the course of a week in March 2016. They analyzed the water for traces of amphetamine, cocaine, MDMA (ecstasy), and methamphetamine. After accounting for the area’s population and the flow of water, they could estimate how much of these illicit drugs were being used.

Their analysis found that Londoners came on top for weekday cocaine use, however they were beaten by the Belgian city of Antwerp when it came to the weekends. The weekends also saw cocaine and ecstasy levels rise sharply, while methamphetamine use tended to remain fairly constant throughout the week.

Generally, western and southern European cities appeared to take more cocaine, although the top consumers of the white stuff were Belgium, the Netherlands, Spain, and the UK.

The researchers also found that MDMA use increased in pretty much all cities during 2016 compared to 2011 levels.

You can check out this interactive map for the full findings.

This method of drug analysis might seem like a gimmick, but as they explain in the video above, it’s actually a highly effective way to study the hidden and stigmatized issue of drug use.

“Wastewater-based epidemiology has demonstrated its potential to become a useful complement to established drug monitoring tools. Its ability to deliver timely data on drug use patterns is particularly relevant against the backdrop of an ever-shifting drugs problem,” Alexis Goosdeel, director of EMCDDA, said in a statement. “By detecting changes in drug use patterns, both geographically and over time, it can help health and treatment services respond better to emerging trends and changing treatment needs. We are delighted that the SCORE group has been able to release data for the first time in the same year as the data-collection exercise."