The 1979 partial nuclear reactor meltdown at Three Mile Island was simultaneously hyper-visible and hidden from public view. It was the subject of non-stop media attention, but its causes and consequences required expert explanation. No fire or explosion marked the moment when insensible radionuclides escaped the facility. Yet, residents recalled a variety of troubling sights, sounds, odors, tastes, and sensations. Public distrust percolated in the interstices between government assertions that little radiation had escaped the facility and residents’ sense memories of (...) the incident. This article traces intertwined networks of activists from Japan and Pennsylvania as they mobilized legally, politically, and scientifically to develop evidence about the offsite effects of Three Mile Island. Exploring the distinct cosmology of evidence that activists marshaled, the article shows how they placed the messy, contingent, dynamic living world at the center of inquiries about the meltdown’s consequences. Activists developed new practices of biological witness that reconfigured the interplay between scientific, legal, and moral authority, while concurrently reformulating sufferers’ subjectivities and notions of scientific objectivity. In the process, they suggested that environmental justice entailed epistemic justice. Their cosmology of evidence served as an argument and a material proof that the beloved but suffering living world, and the sciences used to understand it, could and should frame the governance of industrial society’s invisible harms. (shrink)