The Mendip Hills stretch from the Bristol Channel to Chewton Mendip and from Wells to Chew Stoke and are an Area of Outstanding Natural Beauty, but not all the beauty is above ground.
The limestone hills are famous for Wookey Hole and Cheddar caves, attracting thousands of tourists to the area.
However, they are also riddled with secret tunnels, chambers and sumps (flooded chambers), many interconnected and rarely, if ever, seen by visitors.
Martin Grass, 63, who works in sales and marketing in the UK and Europe, is a warden in the Mendip Cave Rescue organisation and digs under the Mendip Hills.
Mr Grass started his caving with the Scouts in 1970 and his love for the activity has not diminished over the years, nor the excitement of being on the edge of discovering a new spectacular cave.
Mr Grass said: "If you are a caver here you are also part of the rescue team.
"We are all volunteers, called wardens, we don't have members like some clubs and we are self-funded.
"We have a lot of technical equipment now, unlike when the club formed in 1936."
A dangerous sport
Caving is classed as a sport which can be dangerous. A lot of specialist equipment is needed for participants to remain safe.
It is physically demanding at times, with cavers often spending hours crawling on hands and knees along tunnels just wide enough to squeeze though.
Mr Grass said: "We dig and explore looking for chambers, for something spectacular and never seen before.
"We also practice cave rescues a lot, sometimes with a human volunteer as a patient, although for deeper rescues we use a dummy."
Delving into the unknown
When not rescuing people stuck or injured in caves, the team continues to dig and explore the concealed network.
Ideally they start near the surface, following tunnels carved by water or digging out the softer material of silt and small stones.
Mr Grass said: "This is the only sport left where you can go somewhere nobody else has been before.
"Apart from maybe the oceans, space is the only truly unexplored area left and I've stood in caves with others and we were the first people to ever set eyes on it.
"More people have stood on the moon and gazed upon its surface than have seen some caves I've been in."
Mendip Cave Rescue's diving group
The first ever UK cave dive was at Wookey Hole in 1935.
Penelope 'Mossy' Powell and Graham Balcombe dived in bulky suits, a hard hat and with an umbilical delivering gas.
The Cave Diving Group was formed in 1946 by the late Graham Balcombe, making it the longest running cave diving organisation and amateur diving club in the world.
Their expertise is called upon across the world and vital with rescues like the Thai cave rescue.
Divers are needed to get past flooded passages, known as sumps, and requires specialist training and kit.
All the specialist kit is owned by the individual. The only thing they can borrow is an air tank.
Mendip club has 20 wardens who are all volunteers.
Only the police can call the wardens for a rescue using a system called SARCALL, also used for mountain rescue teams.
Some wardens have a Casualty Cave Certificate, allowing them to give pain reliving drugs into the muscles of casualties.
They have a unique radio system, HeyPhone, developed by cavers that works deep underground. It can be affected by iron ore though.
They use another system called Cave Link, which works like text messages, when the cave is noisy or too deep for the Heyphone. It also ensures accurate information is passed from surface to cavers.
Three of the six UK divers involved in the Thai cave rescue in 2018 are involved in Mendip Cave Rescue.
John Volanthen, Chris Jewell and Connor Roe supplied expertise and equipment.
There is no funding from Government or agencies for cave rescue groups. They fund themselves.
A cave, known as the Frozen Deep, was discovered by Mr Grass and his team in 2012.
It was found after years of further exploration off the Reservoir Hole, near Cheddar, which was discovered in the 1960s and, at the time, was thought to be the largest in the Mendips.
Mr Grass and the group continued to dig and explore, leading to the discovery of a massive chamber in 2012.
It became clear as the team entered the cave they had discovered something spectacular.
Mr Grass said: "The day before we entered the chamber we were stood by a dark drop. We had finished digging for the day and I said if there's nothing beyond this drop, let's call an end to this.
"There was a draught coming from the dark, which is a hopeful sign that there's something there. As I stood looking into a 12 metre dark drop I felt like a lad at Christmas, expectant of something exciting the next day.
"We had been digging a small tunnel, on our hands and knees, since 2008.
"It took four years to get to this point."
Exciting discoveries
When the group returned the next day to enter the darkness of the drop, excitement grew as more of a breeze was felt the lower they went.
At the bottom of the 12 metre drop they found another small tunnel to crawl along that led them into a massive chamber.
Mr Grass said: "As we entered the lights showed us huge white stalagmites about five metres (16 feet) high. They looked like they were made of ice, hence why we called it the Frozen Deep. There were huge stalactites as well.
"I stood there with the others and thought, we are the first people to ever see this.
"More people have stood on the moon and gazed at its surface than have seen this.
"We had worked for four years toward this, not knowing it was there. It was well worth the wait and work."
The following week a team of six explored the chamber fully.
The Frozen Deep is believed to be the fourth largest chamber, in floor area, in Britain.
Its spectacular stalactites and stalagmites are also believed to be among the biggest in Britain.
Mr Grass said: "In the last 20 years we have discovered a number of caves over 30 metres and 16 miles of passages in the Mendips.
"Swildon's Hole is about five miles long, has about a dozen sumps, the last few of which need diving bottles to get past.
"Desolation Row is really deep. If you are fit it can take three and a half to four hours to reach there."
Rescue hazards
Mr Grass explained that if someone became injured in the Desolation Row series of caves it could take up to four hours for Mendip Cave Rescue to be made aware of the casualty from another caver.
After that, it can take another hour or so to gather the rescue team and equipment with a four hour descent to reach the injured person.
That's ten hours before the casualty, who may have a broken leg, is reached.
Several wardens are trained to give certain medicines to offer pain relief in the caves as it may not be possible, or too dangerous, for paramedics to reach them.
By the time the casualty is strapped to the rescue board, medicated and returned to the surface for medical help, almost 24 hours may have passed.
There is no break for the rescuers who are working by torch light in damp conditions.
Wardens of Mendip Cave Rescue come from a variety of backgrounds.
Currently they include a doctor, a solicitor, military personnel (one officer and one corporal), a teacher and farmers with their sons.
You have to be 18 or over to join.
If you want to find out more or experience caving, contact Mendip Cave Rescue via Facebook.
Advice to cavers
You should not go into a cave on your own, even if you've told someone where you are going.
If you want to see places very few others have seen then join a cave club and do the training.
Mobile phones do not work underground, the Heyphone is a specialist piece of kit designed by cavers to work underground.
If you believe someone may be lost or stuck in a cave call 999 and ask for the police.
Only the police can call out the wardens.
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