Twenty-five years ago today, reactor number 4 at the Chernobyl nuclear power plant in what's now Ukraine melted down. Some 600,000 workers were exposed to massive amounts of radiation and the radioactive plume caused sickness and death elsewhere in the northern hemisphere. In the months after the disaster, a massive sarcophagus was built over the power plant. But the core is still molten, the sarcophagus is starting to crumble and there's concern that its collapse could release another radioactive cloud. Eben Harrell is a writer and reporter in the London bureau of Time magazine.