About a third of U.S. residents have been receiving tap water containing a previously unidentified chemical byproduct, a new study has found. Some scientists are now concerned — and actively investigating — whether that chemical could be toxic. 

Plumbed drinking water in developed countries is pretty clean, but invisible contaminants can still lurk. One mysterious “phantom chemical” has haunted drinking water for decades, and now researchers have identified it – and found it’s completely new to science.