Thirty years ago an explosion at Chernobyl nuclear power station in Ukraine caused the worst nuclear accident the world has ever seen.
The radiation cloud released spread across Europe, and affected hundreds of farms in north Wales.
Reporter Eifion Glyn filmed the initial aftermath for Y Byd ar Bedwar.
Now, he's been back, and has discovered claims that far more died as a result of the disaster than previously admitted by the authorities.
Eifion explored the abandoned villages in the exclusion zone surrounding the reactor, and met some of the people involved in the catastrophe, to try and find out the truth about the number of people who died as a result.
The true scale of the damage is still the subject of fierce debate today.
Back then, the Soviet authorities insisted that only around 50 people died in the wake of the disaster, while ecological organisations like Greenpeace believe that the accident was the cause of thousands of deaths.
In the programme Y Byd ar Bedwar: Cysgod Chernobyl, Eifion also goes on the search for several people he interviewed first time around.
One of those people is Tatyana Lukina, one of the founders of the Mothers of Chernobyl campaign group, who battled with health problems and family tragedy since 1986.
For nearly 10 years, a consortium of companies has been building a New Safe Confinement structure above the crumbling Soviet era sarcophagus containing the reactor.
The project employs 3,000 people from 23 countries.
Amongst them is a Welshman.
John Morgan is originally from Pembrokeshire, and one of the managers tasked with building the largest moveable structure on Earth.
He revealed the project has another Welsh connection.
The 650,000 bolts holding the structure together are made by a company based in Wrexham.
Watch Eifion's report for ITV News below:
You can see the whole programme on S4C on Sunday April 24 at 8pm.
The programme has English subtitles.