In my hands is a square slab of marble on the end of a long rope. Called a skandalopetra, it is a centuries-old Greek sponge-diver’s weight.

It is impossible to know how many divers were pulled down to the bottom of the Aegean Sea by this stone. And how many didn't make it back to the surface. But I imagine their fingers gripping its rounded edges as I hold my arms out, tip forward and plunge off the boat’s prow and into the blue.

This traditional diving technique dates back to the time of Aristotle. Fishermen used it to reach depths of more than 30m (100ft), where they harvested valuable sponges for as long as they could hold their breath.

I struggle to grip the slippery 13kg (29lb) lump as it drops through the water. When it thumps on to the soft sand and waving seagrass on the sea-floor 15m down, I wrap the rope around my arm and give a couple of firm yanks. On this signal, I’m hauled back to the surface – and my next breath – so shoulder-dislocatingly fast I feel like a marlin being reeled in.

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I am visiting the Dodecanese sponge-diving island of Kalymnos, learning to explore the underwater world on a single breath with an international group of freedivers.

The sport of freediving – diving without breathing apparatus – is the modern evolution of ancient skin diving. And the skandalopetra, a national treasure on loan from the island’s museum, is a physical link with those courageous early divers.

“That’s the most fun thing I’ve done all year!” beams Anna von Boetticher as she surfaces from her first skandalopetra dive. Anna, an accomplished scuba diver, has been freediving for two years. But like all the instructors on this course, she is part-fish, her body trained to perform mind-boggling underwater feats. The Berlin bookshop owner has already reached almost 50m, earning her a place at the world championships.

It is sometimes said that scuba diving is about what is happening around you and freediving is about what is going on inside. Meditative and introverted, it is the opposite of an adrenalin sport. Instead it is about relaxing, banishing panic and conserving oxygen. The calmer you are, the longer and safer your dive. When you are 20m away from your next gulp of air, a pulse-quickening squirt of adrenalin is, frankly, unhelpful.

That may sound frightening, but our bodies come equipped with a sophisticated repertoire of responses to submersion, including an automatic slowing of the heart rate. Some scientists believe this “mammalian dive reflex” points to an aquatic phase in human evolution. Watch a top freediver gliding through the water and the theory is not so hard to accept.

“Freediving is about redefining your own personal limits, getting to know yourself, your body and learning more about your physiology,” says London IT expert Simon Reid, a leading instructor and former British team member.

This fortnight-long freediving summer school has become an annual fixture since organiser and world-class instructor Emma Farrell was invited here as a guest of the first Kalymnos International Diving Festival in 2004. Established to attract leisure divers, the festival is a celebration of the rich diving culture of Kalymnos. Its divers declare themselves the world’s bravest and best. But their bravado has proved costly. Commercial pressures and a risk-taking mindset left many dead or damaged. Even today it’s not unusual to see a former diver limping along the waterfront with a stick.

Kalymnos was once the wealthiest island in the Dodecanese and the centre of Greece’s sponge industry. Hundreds of boats sailed from Pothia, the island’s capital, in search of “Kalymnian gold”. Today, thanks to overfishing and synthetic sponge manufacture, only five local boats operate.

“Kalymnians have a big problem without sponges,” says Sakellaris Atsas, owner of the Kalymna sponge factory on the outskirts of Pothia. “The industry was worth millions of dollars so people didn’t think, ‘What if something happens; what are we going to do?’ So now they start with tourism in the past 15 years.”

That late start means, compared with many Greek islands, neighbouring Kos for instance, Kalymnos remains relatively unspoilt. A small, friendly, low-key island, its rocky limestone terrain and underwater beauty attracts climbers and divers rather than the ouzo and UV crowd. The port of Pothia, a jumble of flat-roofed buildings and narrow back-streets with good cafés, restaurants and bars, has barely grown since the sponge industry’s heyday.

Our mornings begin on the quayside as we board the dive boat that serves as our floating classroom. Much of the course is so-called “line work”. Clipped by wrist lanyards to weighted ropes, we work on technique, safety drills and depth. Wearing elongated fins for more powerful kicks, we descend slightly farther with every dive. I eventually manage to fin down to 16m, where the end of the line is marked with a plastic plate. Something is scrawled on it in a black marker: “The only way is up!”

Freediving, inevitably, has its dangers: the biggest being blackout from oxygen starvation. The golden rule is never dive alone. Why is perfectly illustrated when one student surfaces from a supervised 20m dive, takes a breath and promptly passes out in front of his instructor. “I didn’t know anything about it,” says George Brown, a London gym manager, once he is safely back on the boat.

To condition our systems to tolerate longer breath holds, we practise in an outdoor pool. Called “static apnoea” this discipline is a mind game. Floating face down, we embark on elaborate head trips to distract ourselves from our bodies’ increasingly urgent reminders to breathe. I imagine walking barefoot along a sandy beach, collecting shells with my son. This pleasant mental stroll lasts for two minutes and 45 seconds, until my fourth involuntary chest contraction persuades me to rejoin the world of air. Abbi Kinghorn, a research scientist and newcomer to freediving, manages three minutes and 20 seconds. “I think I am just good at relaxing,” she says.

People freedive for different reasons. For some, it’s a physical challenge; others are on a journey of self-discovery. For me, it is about the strange sense of peace to be found down there. What should be a hostile, alien environment feels natural and pure. Without the rasping of scuba gear, you are submerged in near silence with the distant wash of the waves above and the tiny clicks of fish feeding on the reef. It’s like flying in a dream. For a minute or two at least.

Need to know

DeeperBlue’s Freediving Summer School courses in Kalymnos (0870 950 6589) runs for two weeks from July 6. Three-day beginner’s courses cost from £299. More information from course organiser Emma Farrell: www.emma-freediver.co.uk.

The Kalymnos International Diving Festival is held this year in July. For more information: www.kalymnos-isl.gr

Getting there: EasyJet (www.easyjet.com) has flights to Athens from £49.09 return. Ferries from Piraeus to Kalymnos take about ten hours. Flights from Athens take one hour.
Staying: Kantouni Beach Hotel (00 30 22430 47980, www.kantounibeachhotel.com) has double studios from £27.

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