Chernobyl used to be a regular, busy city like any other, and it was home to the largest power plant in all of Europe.
One day in one part of the factory, one of the workers weren't paying attention when one of the reactors exploded because of too much air pressure.
After the reactor exploded, toxic nuclear gases got released into the air and it got everywhere.
One problem was that this accident happened at around midnight, so many people were sleeping at the time. People didn't notice after a while.
The nuclear gases started to spread everywhere throughout Europe and many people died to exposure to radiation. Being exposed to a lot of radiation can kill someone because it is toxic.
The gases spread all throughout Europe and caused one great problem. It severely affected some countries while some barely got affected but still affected them. Chernobyl is now considered a ghost town and what happened there is considered the worst nuclear accident in the world