Documents seen by BBC News show concerns at the Welsh government and UK Coal Authority that Merthyr Tydfil’s Ffos-y-Fran mine may be abandoned.
Estimated clean-up costs for the site – the size of 400 football pitches – have grown to between £120m and £175m.
Site operator Merthyr (South Wales) Ltd said it was in “constructive dialogue” with Merthyr Tydfil council.
Union Unite, which represents 115 workers being made redundant as coal mining stops, said it understood the company was “committed” to restoring the site in future and would not walk away.
Welsh mountain pony breeder Roy Thomas, 80, who lives metres from the mine’s boundary, called it “a total blight” on his life for the past 16 years and described the mine as a “neighbour from hell”.
Today a giant pit – which is about 656ft (200m) deep – remains.
Mr Thomas described the scene as “absolutely disgusting”, adding that the mounds of spoil material should go back in the hole as there is “millions of tonnes of material in there”.
Ffos-y-Fran has produced nearly 11.25m tonnes of coal since opening in 2008, and is responsible for 86% of the UK’s total coal output.
Since September 2022, its owners have been digging without planning permission – an application for more time was refused and the firm appealed an enforcement notice before announcing a closure date of 30 November.
Negotiations between the firm and the council continue over what happens next.
The company has admitted “insufficient funds” had been set aside to carry out the agreed restoration work.
In a letter released under the Freedom of Information Act to campaign group Coal Action Network and shared with BBC News, the UK Coal Authority’s chief executive criticised the council’s approach.
Writing to the Welsh government on 20 October, Lisa Pinney said there had been “very little visible progress” in preparing for the mine’s closure and “no agreed revised restoration plan or emergency response plan in place if the site should be abandoned”.
How to manage rising water levels is one issue yet to be resolved, with Ms Pinney writing that, without a clear plan, “there is a clear risk to public safety and to the environment”.
