Work is underway to uncover a series of elusive areas at Wookey Hole Caves in Somerset, UK, which staff say have been seen by fewer people than have stepped on the surface of the moon.

Discovered by cave divers in the 1970s, the series of “spectacular” caves will be uncovered after demolitions experts blast a 70m (230ft) tunnel through the solid-rock wall uncovering the cavern – known as Chamber 20 – as part of a £4m expansion project.

“Our customers are always asking us if are there any more caves they can see. There are, but you have to be a cave diver to get to them.” said Wookey Hole director, Daniel Medley, speaking to AM2. “We want to put the tunnel in to open up more of the cave system, to make it one of the largest accessible cave systems in Europe.”

Somerset-based companies Matt Durbin Associates and Demrock are acting as tunnelling crew and explosive engineers respectively, with specialist consulting engineers already having carried out a full geotechnical survey of rocks and plans ahead of submitting for planning approval.

“The cave work is going to cost well over £1m, but it’s hard to know (the exact figure) because we don’t know what we’re going to come across,” continued Medley. “If we’re blasting into what we think is 70m of solid rock, it might not be solid rock. We might be blasting into another huge cave or a big void so we might have to spend hundreds of thousands of pounds on gantries and safety work. There’s no way of knowing until you get there as you go along. We’re really blasting into the unknown.”

With work already underway, the tunnelling team have a very small window to able to open up the chamber because of rare bats inhabiting the cave. At the end of June, the bats’ mating season starts and at that point the creatures aren’t allowed to be disturbed.

“We’re going to try and keep it as natural as possible,” added Medley. “We have our own team on-board for design. We want to make it look like a natural cave environment. The rest of our caves are very much lit up with LED technology, but this one we want to keep as natural as possible.”

Work is expected to take around a month to complete. Wookey Hole is hoping to increase visitor numbers by around 20 per cent once the addition is open to the public – representing a rise from around 250,000 visitors a year to 300,000.