At this hour 40 years ago, the chain of events that led to the worst nuclear accident the world has yet experienced were already underway, as an ill-fated low electrical power capacity drill slowly advanced, in northern, Soviet-controlled Ukraine. The growing technical threat flashed into crisis when Reactor No. 4 at the Chernobyl Nuclear Power Plant near Pripyat, Ukraine, exploded at 1:23 a.m. April 26, 1986. This detailed graphic looks at causes, the damage done, the dangerous aftermath, and efforts to safely contain the debris.