If you or any of your friends have gotten sick with COVID-19 recently, you’re not the only ones: Case rates are rising in the Bay Area and the proof is in the pipes. According to the California Department of Public Health, the region now has the most viral wastewater than anywhere else in California.

In an email to SFGATE, Amanda Bidwell, a wastewater researcher and data analyst at Stanford, said that over the past 21 days “consistently high concentrations” of SARS-CoV-2 have been detected in wastewater samples collected across San Francisco.

“Currently we are seeing some of the highest concentrations we’ve ever measured” at these locations, Bidwell continued. And across San Francisco, the levels are approaching those last seen in December and January. Because individuals shed the virus before getting tested, this method of monitoring wastewater helps predict upcoming surges while including data from those who are asymptomatic.