At the Soviet Union's Chernobyl nuclear power plant, the plume of smoke is gone, but a cloud of uncertainty still obscures the cause of the severe accident there. That accident, which began early on Saturday, 26 April, caused an intense fire that lasted many days, melted unknown quantities of reactor fuel and released into the environment a large fraction of the reactor's inventory of volatile radioactive fission products (including perhaps a million times more iodine‐131 than came from Three Mile Island). On 11 May Soviet officials reported that they had stabilized the reactor and had successfully prevented the core from melting through the bottom of the containment building. Although the crisis is past, formidable tasks lie ahead. These chores include entombing the severely damaged core; isolating or decontaminating an area more than 30 km in radius around the plant, from which about 100 000 people have been evacuated; monitoring radiation levels over far more extensive areas; and determining the cause of the accident to prevent repetitions elsewhere.