People have been paddling around Thailand’s numerous shallow coral sites with goggles and snorkels for decades, but it was not until about fifteen years ago that more serious diving with proper underwater breathing apparatus really began to take off. At that time, the majority of dive shops in the kingdom ran businesses teaching basic ‘discover scuba’ and open-water courses.

As time passed, word spread of Thailand’s superb dive sites, and dive shops became increasingly competitive in the support they were able to offer advanced divers. This included offering specialized mixed breathing gases, larger volume diving tanks, double tanks, auxiliary tanks and underwater scooters. As these resources have become more readily available, seasoned divers have been able to open the door on a whole new world of exploration. An intrepid breed, they are often referred to as ‘technical divers.’

Respiration is the over-riding concern in diving. The human body is profoundly affected by the compressed gases breathed underwater. Air normally contains about one-fifth oxygen to four-fifths nitrogen — gases which when breathed at surface level are perfectly normal. But once a diver passes beyond normal recreational dive limits, they become potential killers.

A diver who descends to 100 meters may be breathing in compressed form ten times as much nitrogen and oxygen as normal. Too much nitrogen at depth can cause narcosis — with similar results to drinking alcohol in excess. If a diver comes up too fast, there is the risk of decompression sickness — popularly known as “the bends” — which occurs when the body is not given enough time for the gas to dissipate. This can be extremely painful and result in paralysis or even death.

Oxygen, which is essential for sustaining life, meanwhile becomes toxic beyond 60 meters and this can induce convulsions — and drowning if the regulator used for breathing through the mouth falls out.

Mixed gases are essential to extending the depth and duration of dives. Ordinary recreational divers breathe compressed air, and do not have to stop to allow for decompression on the way up. Technical divers use nitrox and tri-mix gases instead. Nitrox has extra oxygen added to the air during compression, and reduced nitrogen, which helps extend dive times and mitigate the risk of decompression sickness.

Tri-mix incorporates helium into the mix, an inert gas that reduces the risk of both nitrogen narcosis and oxygen toxicity. For deep dives of more than 60 meters, technical divers use tri-mix to go down and nitrox for decompression when they come back up.

Two of the most popular new activities in Thailand are wreck diving, mostly in the Gulf of Thailand, and cave diving, mostly along the Andaman Sea coastline. The Gulf of Thailand falls well short of being an Asian Bermuda Triangle, but is rich in sunken wrecks resulting from misadventures in trading, piracy and war. Many stories have been lost in the mists of time but, according to one list, there are at least 179 sunken Japanese ships — or marus, as some divers prefer to call them.

Some of the biggest recent discoveries date from the Second World War. In mid-2005, a group of technical divers from Koh Tao came across a US submarine that had been sunk in 72 meters of water by the IJN Hatsutaka, a Japanese minelayer that recorded an anti-submarine action with depth charges at the time.

The wreck turned out to be the remains of USS Lagarto, SS 371, sitting virtually upright on the seabed. The sub was completed in May of 1944 in Manitowoc, Wisconsin, on Lake Michigan — a city today twinned with Kamogawa in Japan. The Lagarto was lost on 3 May 1945, taking down 86 crew. Its exact location remains a secret known to very few. A year after its discovery, however, US Navy divers from the Salvor (ARS-520) were able to confirm that the wreck was indeed the Lagarto. Relatives have been informed, and a memorial plaque and flag erected on the wreck.

Although the USS Lagarto lies well beyond the reach of almost all divers, and is a war grave that like all wrecks in Thai waters must not be disturbed or pillaged, there were at least naval records on both sides to cross reference. For many wrecks, nothing of the kind exists and curious divers often fall back on questioning local fishermen. With their special local knowledge, fishermen may have noticed that schools of fish are more abundant or behave differently in the vicinity of wrecks. Wrecks also sometimes get noticed when they snag nets.

The Gulf of Thailand has many pottery wrecks from sunken merchant vessels, some of which date as far back as 700 years. Siam, as Thailand used to be known, had a more inland capital before Bangkok at Ayutthaya that was readily accessible by boat up the broad and majestic Chao Phraya River.

It was therefore a major trading center in its day, attracting merchants and adventurers not just from Asia but as far afield as Europe. The waterborn traffic ebbed both ways in ancient Siam, and captains often placed pottery items in the bowels of their vessels for ballast and stability. They could never have envisaged the value such items would acquire in the years to come.

Non-divers can get a good idea of the richness of these underwater artefacts by visiting the Underwater Archaeological Museum at Khai Nern Wong, an 18th century fort built by King Taksin on the outskirts of Chanthaburi on the Gulf of Thailand’s eastern seaboard. The collection, which is open Mondays to Saturdays, includes rare Sawankhalok pottery as well some ancient guns.

The museum provides a rich glimpse of the archeological riches of Thai waters, but it is probably the kingdom’s natural treasures that are proving a greater test of diving skills. Thailand has a growing reputation among cave divers due largely to the limestone caves found all along its western coast in the Andaman Sea.

Divers experienced in the use of tri-mix can enjoy some major challenges here. Indeed two of the largest caves so far discovered in Asia are in the vicinity of Krabi province, on the mainland near the island province of Phuket.

Sre Keow was one of the first to be explored. It is accessed through a pond of about 15 meters in diameter that was once mainly used for washing elephants. A small entrance at a depth of ten meters leads into one of the deepest caves so far discovered in Thailand.

It is uncertain how many caves suited to diving exist in Thailand since new ones are being discovered all the time in the south, and also in lakes elsewhere. Another spectacular known cave, Song Hong, is shallower than Sre Keow but offers much deeper penetration into the rock formation. It is accessed through a larger 75-meter pond which sinks to a depth of 110 meters. About twelve meters down, a relatively small entrance provides access into a vast cave — so large in fact that a light beam will not reach the cave wall on the other side.

Divers descending into the cave find survey lines fastened securely about every ten meters. They feel their way down along the primary line to a depth of about 120 meters, at which point the passage becomes more horizontal until it reaches a depth of about 140 meters.

The longest penetration of the cave so far from here onwards at the same depth is a remarkable 800 meters. The total dive time for such a feat is about six hours, and it requires a major team effort of typically two or three people. Extra tanks are staged along the route. The bottom diver uses twin rebreathers and a two-man diver propulsion vehicle (DPV) to carry him along.

Support divers are essential to such a dive, and equipment redundancy is a major preoccupation. Two and even three pieces of each type of equipment are often required for such a perilous undertaking. Should a rebreather flood at this depth and penetration, the only way for a diver to survive would be using an open circuit tank that has been specially staged, or a special bailout tank that must actually be carried alongside.

“Breathing the wrong gas at the wrong depth can kill you in a matter of minutes,” comments Henny Beeber, an American businessman resident in Thailand who is also a highly experienced technical diver.

So why do people take such enormous risks in the name of recreation?

“I was the first person to reach a sunken US P-38 bomber in the Gulf, yet thousands of people have been to the top of Everest,” explains Beeber. “I am your average Joe, and the only way I could ever have got up Everest was if three sherpas carried me. Nothing can beat the sense of achievement and adventure diving gives me.”