In 2008, Spain began filling 300-metre-deep coal mine with 547 billion litres of water; four years later, the site looked nothing like it once did

For nearly six decades, coal mining transformed a part of Galicia in northwest Spain into an enormous open pit. The excavation at As Pontes eventually reached more than 300 metres deep, leaving behind a vast landscape scar after mining operations ended.

But instead of leaving the abandoned mine untouched, engineers embarked on an ambitious rehabilitation project. The enormous pit was deliberately filled with water, eventually creating what became one of Spain's largest artificial lakes.

The transformation took more than four years and involved around 547 billion litres of water. Today, the former coal mine has a lake, beach, nature trails and new habitats, offering an unusual example of how a heavily mined landscape can be repurposed.


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As Pontes Coal Mine Was Once One Of Spain's Biggest

The story of As Pontes began in the 1940s, when lignite, a form of brown coal, started being extracted from the area.

By 1972, the mine and its adjoining power station were owned by Endesa. The company became a major part of the region's economy, with the mine supplying fuel to the nearby coal-fired power plant.

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Over the following decades, around 260 million tonnes of lignite were extracted from the site. The scale of the operation resulted in a huge open pit measuring more than 300 metres in depth.

The mine eventually closed permanently in 2007 as environmental regulations and changes in Europe's energy sector put increasing pressure on older coal-fired facilities.

Why Lignite Mining Creates Such Huge Open Pits

Lignite is often described as brown coal and is considered a relatively low-grade form of coal. It contains less energy than harder forms of coal, meaning larger quantities may need to be burned to generate the same amount of electricity.

Lignite deposits are also frequently found relatively close to the surface. This makes them suitable for large-scale open-pit mining rather than conventional underground extraction.

The combination can result in enormous excavations, with mining companies removing vast quantities of earth to access the fuel underneath.

What Happened To The Mine After It Closed?

The closure of the mine did not mark the end of the site's story.

Instead, rehabilitation work began to transform the enormous excavation. According to Endesa's account of the project, the former mining area became the focus of a long-term environmental restoration programme.

One of the biggest challenges was deciding what to do with the enormous pit itself.

The answer was extraordinary: fill it with water and create a lake.

How Engineers Turned A Coal Pit Into A Lake

Filling a 300-metre-deep mine pit was far more complicated than simply allowing rainwater to accumulate.

Engineers had to consider where the water would come from, how quickly the pit could be filled and how the changing water levels would affect the surrounding environment.

A 2023 engineering study titled Environmental recovery of As Pontes mine pit lake, published in Boletín Geológico y Minero, examined the rehabilitation and filling of the mine.

Researchers Francisco Rivas, Javier Samper and Luis Montenegro from the University of A Coruña analysed the process and its environmental evolution.

According to the study, the filling process was completed on 18 April 2012, more than four years after it began. Around 547 billion litres of water were pumped into the former mine.

Why The Artificial Lake Has Two Different Water Layers

One of the most interesting aspects of the As Pontes lake is what happened beneath its surface.

Research published in Mine Water and the Environment examined the hydrochemical evolution of the lake as the former mine filled with water.

The study found that the lake developed two distinct water layers separated by what scientists call a chemocline.

The upper layer is relatively oxygen-rich and only mildly acidic. The deeper layer, meanwhile, contains very little oxygen and is considerably more acidic.

The researchers linked the formation of these layers to differences in the sources and chemical characteristics of the water, along with conditions such as temperature and mineral content that affected how the water mixed.

In other words, what looks like one enormous lake from the surface has a surprisingly complex environment underneath.

From Open-Pit Mine To Artificial Lake

The transformation did not stop once the pit was filled.

Today, the lake covers around 865 hectares and forms the centrepiece of a much wider restoration project.

According to Endesa, different habitats have developed around the shoreline, including meadow, scrub, woodland and wetland areas. Wildlife has also returned to parts of the landscape that were once dominated by mining infrastructure and waste rock.

The former industrial site now includes nature trails and a sandy beach, while recreational activities such as kayaking and canoeing take place on the lake.

As Pontes Lake Became A Tourist Attraction

The transformation has also changed the relationship between the site and the local community.

For decades, employment and economic activity in As Pontes were closely connected to coal mining and power generation. After the mine closed, the community faced the challenge of finding new uses for the former industrial landscape.

The restored lake has since become a recreational and tourism destination. It has also received Blue Flag certification, reflecting standards covering areas such as water quality and environmental management.

The site therefore represents more than the conversion of a mine into a body of water. It has become part of the area's attempt to develop a post-mining economy.

Why The As Pontes Mine Lake Is Important

As per a TOI report, As Pontes is part of a much larger global story. Mining has left enormous open pits in countries across Europe, North America and Australia, and many former mining sites still face environmental problems long after extraction stops.

Some abandoned mine lakes can become highly acidic or contaminated, depending on the geology and minerals exposed during mining. Others require careful monitoring and long-term management.

The experience at As Pontes demonstrates why the rehabilitation of a mine cannot simply end when extraction stops. Water chemistry, groundwater, surrounding ecosystems and the future use of the land all need to be considered.

Can Old Coal Mines Really Become Lakes?

Not every abandoned mine can be transformed in the same way. The geology, water sources, chemical composition of the surrounding rocks and local environmental conditions can all determine whether a pit lake is viable.

At As Pontes, engineers spent years managing the filling process and monitoring how the water evolved.

The result is an unusual example of large-scale mine rehabilitation: an excavation created by decades of coal mining is now a vast artificial lake surrounded by vegetation, wildlife and recreational spaces.


Inputs from TOI