CHERNOBYL, Ukraine—The disaster at Chernobyl on April 26, 1986, is currently ranked as the worst nuclear accident in history. Officially, tourism has opened up here, but areas remain that are too dangerous for tours. On the eve of the event's 25th anniversary, Scientific American frequent contributor Charles Q. Choi traveled to Chernobyl and nearby Kiev to explore the site as well as report on research going on there via a series of stories, along with these images.

View a slide show of the Chernobyl nuclear disaster site.