The Eden Project plans to save a gigantic “cathedral mine” by turning it into a £30 million tourist attraction, bosses have revealed.

The Cornwall botanical site, which has welcomed more than 18 million visitors since opening in 2001, is looking to transform a man-made cavern that lies below the tied Isle of Portland on the English Channel.

It is hoped that the Isle of Portland, which features towering slabs made of limestone that formed 145 million years ago, will form the basis of a new ticketed Eden Portland attraction.

Planners say that the caverns would be turned into an “underground cathedral”, which would contain exhibits on the topics of nature and biodiversity, evolution, and extinction.

Efforts to secure the required £30 million of funds from reserves, donations and ministers are being led by Sir Tim Smit, co-founder of the Eden Project.

The man-made cavern, as it looks now, would contain exhibits about nature and biodiversity, evolution, and extinction

Credit: Mark Godden

“It would be a hugely successful, world-class destination that is both a visitor destination as well as a scientific institution,” Sir Tim said.

“The location is to die for. It’s beautiful and it’s beguiling. I have zero doubt about this project, which is what Portland needs.”

The limestone that can be found on St Paul’s Cathedral was taken from the cliffs of the Dorset site, and use of the stone on buildings dates back to Roman times.

However, Sir Tim added he is concerned that the cost of funding for the Portland project may not be met, as time constraints mean work would need to start as soon as possible.

Another artist's impression of Eden Portland, which is facing a race against time to secure £30 million in funding

“If we don’t get the support we need, I’m afraid we are just going to have to go. We’ve got other things to do,” he said.

“But, you know what, we will take the Portland idea and we will put it somewhere else because the idea is so good and we haven’t got time to wait.”

Sebastian Brooke, project director of Eden Portland, said the plans are particularly timely because the world is currently “on the crest of a breaking wave of a mass extinction event”.

Previous ideas for the site have included a dinosaur museum and an observatory, which would have charted the mass extinction of species, although both of these were withdrawn due to insufficient funds.

The Eden Project is also set to open a new £85 million visitor attraction, which has the provisional title of Eden Project North 1, as soon as next year in Morecambe.