A mysterious lineage of the COVID-19 virus – containing a large and startling collection of mutations — has appeared in California’s wastewater system, proof that the fast-moving pathogen is continuing to test new survival strategies.

Oddly, the virus has been found only in poop, not people. First detected in an undisclosed community by UC Berkeley scientists, it has shown no signs of causing illness. It’s led to no new outbreaks.

So where is it coming from? And should we worry about it? No one knows for sure.