Twenty-five years ago today, the sky fell on a corner of Ukraine. The Chernobyl nuclear plant, a symbol of man's triumph over the atom and the pride of a nation, experienced a system failure so complete, so devastating, that it left an irreparable wasteland in its wake. Explosions rang out. Radioactive smoke stained the early morning sky. And 350,000 people lost their homes forever.
It started with a systems test. The Chernobyl plant had been operational for two years without a solid backup plan in the event of an electrical power outage; the generators in place took 60-75 seconds to reach the capacity needed to run a main cooling water pump, far too long to meet safety requirements. So, as the first step in an experiment that rerouted electrical supplies to Reactor Four, engineers gradually decreased its power output.
Things started to go wrong. The reactor's output dipped much lower than anticipated, poisoning the reactor core. Alarms rang out, largely ignored. And though the experiment was completed as planned, it left behind a wholly unstable system. So much so that when an emergency shutdown was triggered - for reasons still unknown - the resulting massive power spike lead to overheating, which in turn caused a catastrophic explosion, the fracturing of fuel rods, and a graphite fire that sent deadly radioactive material straight into the air.
Chernobyl workers died within minutes; the fire inside Reactor 4 burned for weeks. Approximately 100,000 square kilometres were contaminated with fallout, spread across Belarus, Ukraine and Russia, and a 30km radius "exclusion zone" still exists. All told, more than 4000 people are likely to die prematurely from exposure to Chernobyl's radioactive spew. It remains the worst nuclear disaster in history; while Japan's recent tsunami-caused Fukushima Daiichi tragedy was also classified a maximum 7 on the International Nuclear Event Scale, the ruin left behind by Chernobyl eclipses even it in both scope and severity. Today, 25 years later, we look back. [Photo credit: Sergey Ponomarev/AP]