Today marks 25 years since the worst nuclear accident in history.  On April 26, 1986, a reactor at the Chernobyl nuclear plant in Ukraine exploded, killing at least 30 people and sending hundreds of others to the hospital.  Soviet authorities evacuated 200,000 people from the area, and set up an exclusion zone the size of Switzerland around the crippled plant.  Millions of people were exposed to radiation, and thousands developed thyroid cancer.  Despite that, the United Nations says there has been no major public health impact in the decades since the accident.  But radiation experts say the true health costs of the disaster are still unknown.

Anecdotal evidence suggests beavers, deer, hawks, eagles and other wildlife have returned to the Chernobyl exclusion zone in abundance since people fled the area in 1986.  But one radiation ecologist says that picture is misleading.  Biology Professor Tim Mousseau of the University of South Carolina says biodiversity in the area around Chernobyl has dropped dramatically since the meltdown.  Last year, he and his colleagues published a census of wildlife in the exclusion zone.  He talked with BackStory about his findings.