The Xing Da was an extreme dive for Brendan O Brien - because he had just watched it sink.

An extreme human adventure, is how record-breaker Patrick Musimu describes competitive freediving. Talk to him about his ultimate feat (a 209.6m dive in the No Limits category) and youll hear him describe it as entering a new dimension... magical moments, and proclaim that the sea whispers to every single one of us when we dive, providing access to hidden treasures.

While I enjoyed my last club outing to St Abbs, there was nothing in my experience that could come close to Patricks vision of the underwater world. But his view set me thinking about the times when my diving has been an extreme adventure.

I may not have entered any new dimensions, but these dives have sometimes taken my breath away.

These magical moments might be the times when the sea does whisper to us - we just have to see our dives through a different lens. When we do, that 30-minute fin around a well-dived wreck could become your own extreme human adventure.

But where do we find these dives, and what distinguishes them as being extreme enough to be described as a human adventure

I started by considering wreck dives - the biggest, least-dived or deepest. Fellow-divEr contributor and wreck expert John Liddiard helped by giving me some ideas about what could be considered an extreme wreck dive.

As far as depth is concerned, we agreed on the wreck of the Jolande in Ras Mohammed.

Its really deep (a worlds deepest wreck dive record was recently set on it at 205m) where the main body of the wreck lies, while at the top of the reef there are parts of the wreck and its cargo that are still within sport-diving limits.

But for me, this wreck site has too many people (I counted 16 dive boats around it last time I dived on it), too much porcelain (its cargo of toilets) and not enough excitement.

I eventually settled on the Rondo in the Sound of Mull, not because of its size or history, but because it is the only wreck in UK waters (or in the world) that has a depth range of 6-51m (the official BSAC depth limit), and where in parts the hull appears to be almost vertical.

My last visit to this wreck was on a mirror-flat morning with excellent visibility; I could even see parts of the stern hiding under waving kelp fronds. With snow on the surrounding mountains, clear blue sky and the smell of bacon and sausages for the post-dive breakfast, this was a perfect pre-dive moment.

Once in the water, the stern to midships area seems to drop off underneath you all the way to 51m, the spot where the bow ploughed into the seabed. This is the narcosis zone, where the current gets stronger, the dark gets darker, and the climb back up the wreck seems like a peak too far.

A few head-clearing metres later and youll start to get a real feel for the majesty of this wreck, not because of its size or condition, but because its lying in front of you like a mountain wall waiting to be climbed.

John Liddiard assures me that the average angle for this wreck is just 35, but at 51m you have no time for numbers - for a wreck, this is as steep as it gets.

There is only one place I know in the world where you can dive the wreck of a Spanish galleon with the guarantee that you will be able to find, pick up and examine a trinket or item of jewellery.

The wreck of the El Buen Consejo off Anguilla is an Underwater Archaeological Reserve, and while the wreckage is spread across a few hundred metres, unrecognisable as a galleon, it is a most unusual example of a wreck remaining deliberately unexcavated.

It is the only opportunity you will ever have to observe medals, coins and jewellery under water that may not have been touched since the ship sank in 1772.

If the El Buen Consejo does not sit with your childhood dream of what a shipwreck should look like, the Arabia in Lake Huron will.

At a depth of 34m, this barque, sunk in a storm in 1884, could be the most perfectly preserved, clichéd example of a shipwreck in the world, because the hull is in one piece and its masts and rigging are intact. The cold (only 2-3C year round) and the depth make this an advanced but challenging dive. Over the years, several divers have died after underestimating the conditions.

With the sinking of HMS Scylla, the UK joined the rapidly growing collection of countries in which divers sink ships intentionally to form artificial reefs. The biggest of these is now the Oriskany in Florida (see separate article), but the extreme of artificial reef sinkings had to be that of the Captain Keith Tibbetts in 1996 off Cayman Brac, when Jean-Michel Cousteau stayed onboard as it disappeared beneath the waves.

Clearly this sort of feat is not achievable for us lesser mortals, but you cant beat diving on a ship only moments after it has sunk.

Several years ago, I watched the cargo ship Xing Da sunk as an artificial reef off the coast of Bermuda. Watching a sinking is an intense experience; the vessel will creak and groan as the sea tries to bend it out of shape, forcing geysers to erupt out of the hatches as the spaces below decks fill with water.

Soon after the Xing Da hit the seabed, safety divers pronounced the new wreck safe to dive. Down below, we witnessed currents trying to push it harder against the reef and causing it to creak more, as plumes of sand from the hold formed billowing clouds. The birth of a wreck is truly an extreme experience.

The diveable quarries and pools in the UK have aimed to create a theme-park effect rather than anything extreme, but if you want adventure (with truck-loads of caution) there can be no doubt - Dorothea Quarry in North Wales is the most extreme inland dive site in the UK.

Its more than 100m deep, disorientating, unmanaged, and has claimed the lives of far too many divers.

A few years ago, I spoke to a police diver who had spent the previous day recovering a divers body from its depths. His team had seen another one they hadnt known about several metres below the one they recovered, but had to leave it there for another day. Theres only so much of this site you can do at any one time - its dark, cold and can freak you out far too quickly, he said.

Dorothea is where deep air diver Mark Andrews demonstrated just how devious and cunning narcosis could be at depths of no more than 30m. During a training course he had challenged me to stay above 30m at all times, no matter what happened. By the time I reached this point, Mark proved how I was suffering from narcosis.

It wasnt just the depth, but the cold, darkness and, eerily for a flooded quarry, what felt like a slight current. Mark waved at me to look at something below 30m and, my head in the clouds,

I complied. Thats how easy it is to slip into the Incident Pit, especially at the UKs most extreme inland dive site. The British Sub-Aqua Club does not support club dives taking place at Dorothea.

There is a sense of wonderment when you experience the free-fall from the surface to the top of a sheer wall followed by the drop down its sides.

At some point you know you cant go any deeper, but still, the inky blue or blackness below beckons (or maybe its the narcosis again).

The Cayman Islands, like many other locations around the world, has sheer walls adorned with beautiful corals and sponges, and every year thousands of divers marvel at them - and dream about what lies below.

For a fortunate few, this dream can become reality through the submarine Atlantis Deep Explorer, in which you, the pilot and one other person can descend to 333m (1000ft) - the depth only the very deepest trimix divers have plumbed.

OK, you dont get wet and you dont need any diving skills, but once you leave the depth limits of sport divers, this wont matter. Youll witness towering limestone haystacks covered in deepwater gorgonians and stalked crinoids, and other forms of life straight out of the National Geographic channel. My buddy on this dive called it the adventure of a lifetime.

Wall diving has its attractions but there is a catch - the top is often a gently sloping plateau. But on an underwater pinnacle or seamount there is drop-off all around you.

For accessibility and spectacular underwater scenery, the seamounts and pinnacles found off the island of Saba in the Caribbean have no match, particularly at Third Encounter, the top of an underwater mountain with a summit starting at 28m. Its only when you are guided out into the blue here that you realise what stunning means.

After a few minutes, the Eye of the Needle slowly comes into view. This aptly named tower is 80m tall, has a diameter of just 15m and a tip at 30m.

Heart pounding, there is just time to circle the top of the pinnacle and admire the massive barrel sponges surrounded by bright corals and sponges.

If walls and pinnacles are about going deeper and vertical, Scotlands Falls of Lora below the Connel Bridge, at the entrance to Loch Etive, are about going horizontal, but at underwater speeds you may never have experienced before.

This might not be the fastest drift in the UK, but if dived at the right time, it is the most extreme adrenalin speed rush youll find in our waters.

John Liddiard and I agreed that this dive could be extreme, depending on how you approach it. He suggests that this mixed-up site should be dived at highwater slack or just after, but for

a true speed and washing-machine experience, Id suggest diving it when the tide is on the flood.

Under these conditions, boat cover and plenty of experience is a necessity - once you catch the incoming tide, it will pick you up and fly you through the channels between large rocks and boulders. Just when you think youre about to hit one of them, the current will whip you around it.

Expect to lose your buddy, and watch out for the downcurrents once you pass the channel, as theyll throw you down to 25m-plus faster than you can clear your ears.

This speed frenzy will almost certainly be followed by unexpected upcurrents, and a strong chance that you will surface 50m away from your buddy while spinning in a mini-whirlpool.

As far as extreme is concerned, this is the only dive I would ever think twice about doing again - approach with caution, and dont confuse the pursuit of extreme diving with sheer stupidity.

While the Falls of Lora will propel you through the water at a speed of several knots, the currents found in the Emerald Sea off Vancouver Island, British Columbia, make Lora seem like a trickle.

At the Nakwakto Rapids, there are regular tidal exchanges of up to 16 knots, and the site is listed in the Guinness Book of World Records as having a recorded tidal current of 22 knots.

No one would advocate diving in such conditions, but what currents of this magnitude produce are dive sites full of colour, and massive forms of marine life. The plumose anemones are among the biggest I have ever seen, and the giant pacific octopus is a creature to be kept at tentacles length.

The best example of extreme coldwater current diving is Browning Wall off Hurst Island, Northern Vancouver Island.

Here we were greeted by black water whirlpooling around the boat as it struggled to move across the sheer cliff face of this site. But once slack arrived, there was a chance to see an explosion of colour that beats most warmwater locations for variety and beauty.

Perhaps more adrenalin-pumping than strong currents are the underwater gales experienced in the waters of the Maldives. Here, gripping hold of the reef to prevent being blown off a ledge while looking out into the open ocean is the norm in the places where the sharks and rays are found.

The deepwater currents hitting the walls of the atoll really do feel like being in a gale. One minute the current is manageable, then for several seconds your biceps get a workout as the sudden rumble (and you really can hear this) of the current tries to rip you off the reef. Its all you can do to look straight out and hope it doesnt get stronger.

On my last Maldives trip I saw a diver lose his grip, causing him to tumble uncontrollably across the reef. He surfaced about a mile from the pick-up point. Some of these dives arent for the faint-hearted.

Visibility forever, was the phrase used by Jacques Cousteau to describe the dives around Ginnie Springs in Florida. Here you can experience vis like nowhere else in the world. Other countries can boast diveable freshwater springs, but for accessibility, clarity and other unusual features, I dont believe this site can be beaten.

This complex is popular with bathers and boaters, but under water there are cave systems. These are off-limits to sports divers, but Ginnie Cavern isnt. At the rear of this cave, in about 15m, is a grate that prevents access to the labyrinth of tunnels and caverns. Not that you could gain access here anyway - its all you can do to hold on to the bars as the force of a daily flow of 35 million gallons presses against you.

You can also witness the extremes of clear water and murky visibility where the spring waters meet the Santa Fe River. Ask at the dive centre, and it will point you to where (depending on the season) you can put your arm into

the river while under water and see it disappear, like something in a science-fiction movie.

If cave-diving as a sport is more extreme than you want, there are plenty of places where caves can be entered safely but still reveal some of the marvels that specialist cave-divers witness.

On the surface of the Big Island of Hawaii, there are locations where you can witness the lava flow pour into the ocean, while at the visitor centres you can see film footage of what happens when it descends below the surface - which is that long lava tubes form, spouting violent plumes of steam, bubbles and volcanic material.

You can dive caves and tubes that would have been born in similar circumstances only a few hundred years ago. Similar to caves at their entrance, they soon become a tighter fit. The walls are encrusted with brightly coloured sponges and home to a variety of unusual light-fearing creatures.

When it comes to extreme marine animal encounters, the shark obviously tops the list. I have only ever seen sharks in the wild, without human interaction, but organised shark-feeding encounters are far more commonplace than they were a decade ago.

We are to bull sharks what a Mars bar is to us - a snack, claims DIVER correspondent Gavin Parsons, referring to the bull shark-feeding experience in Santa Lucia, Cuba. He describes these creatures as, one of the most dangerous sharks which regularly attack humans.

And as far as their feeding habits are concerned, they are scavenger feeders and will take anything... next to one, a person is very insignificant.

For Gavin, fear was the biggest challenge, followed by the depth - the dive is at 27m. He also had to keep a close watch on the shark numbers: We had eight... they tried to circle around me, and seeing as there was the feeder, me and just one safety diver with a metal prong, it could all have gone terribly wrong.

So whats the attraction For Gavin: This is the only place these days where you can effectively get up very close and personal with bull sharks. They make grey reefs and Caribbean reefs look like poodles.

Fortunately you dont have to worry about whats creeping up behind you, as normally the divers are controlled and placed out of danger with their back to a wreck, so the sharks cant get to them without being seen.

Elsewhere, there are larger marine creatures with which to interact, such as whale sharks or basking sharks or, if youre lucky and spend enough time in the water, you might see wild dolphins or even humpback whales.

But if you dont want to leave this kind of dive to chance, there are places where you are pretty much guaranteed interaction with rays and sea-lions.

Of all the cleaning stations and feeding spots for manta rays Ive seen around the world, none is as breathtaking as Manta Ray Village off Hawaiis Big Island. Here they are at their most agile, fast and acrobatic in their behaviour.

They come to feed on the clouds of plankton attracted by the lights of the Kona Surf Resort, but our boat had laid down underwater lights of its own, and after a few hours there was enough plankton to bring these giants in for a feeding frenzy. Its hard work keeping the light from your torches on them as they dive, swoop and somersault at remarkable speeds.

I have dived other locations in the world where sting rays are common, and where they will sometimes interact with you, but there is nowhere as extreme as Stingray City in Grand Cayman. In just a few metres of water, the rays wrap themselves around you in their desire to be fed.

It all started with fishermen throwing out scraps while cleaning their catch on this sandbar. Now you can draw on the rays desire for food by holding a piece of squid in your hand and guiding them around you. Eventually you give in before being enveloped by hungry rays.

You can swim with seals around the UK, but even in places such as the Farne Isles diving with a seal audience can be a rare occurrence. However, in the Sea

of Cortez, Mexico, off the island of Los Islotes, you are sure to come face to face with one of the many California sea-lions that live around the boulders at the base of this small guano-covered pinnacle.

Theyre a feisty bunch and protective of their territory. They consider bubbles a sign of aggression, so you need to approach with caution. Despite this,

they swim faster and have a tendency to swim up to you and, at the last minute, veer off at speed while snorting air through their nostrils at you.

The dive guides will say they are at play, but halfway into our dive they started to get more than playful as they battled with each other. None of them was hurt, and this seems to be how they sort out disputes, but to be amid the action was exhilarating.

Youll also come across California sea-lions around the Channel Islands off the coast of California, but the real reason why divers make the overnight journey to remote islands such as San Clemente are the underwater kelp forests, where giant kelp grows to the surface from as much as 30m down.

Youll find similarly extreme kelp forests in Tasmania, but while these are attractive for the unusual marine creatures that hide within their branches, they cant compare with the majesty of the Channel Islands forests. Hiding among the trees you might come across blue sharks, sea-lions and rays. Youll also find that many divers here are underwater hunters, armed with spearguns and slings.

Seeing a camouflaged diver stalking and eventually shooting some unlucky fish is a shock to the senses. Culturally this practice is widely accepted on the west coast, and to ensure that stocks arent depleted, its also highly regulated, with divers required to obtain a licence and to adhere to rules about what can and cant be hunted, depending on the time of year and current stocks.

If hunting for your dinner using scuba apparatus horrifies you, the alternative is to catch your quarry using freediving skills. Patrick Musimus depth record was achieved by partially filling his air cavities with sea water, and was so extreme that it may well remain unchallenged for years. Illusionist David Blaine recently tried but failed to beat the nine-minute world breath-hold record.

Most divers would probably regard such extreme activities as out of reach but, to borrow freediver Tanya Streeters slogan, any divers who are reasonably fit can redefine their limits.

I suspect that many freedivers just feel quietly smug in the knowledge that they have achieved what most people couldnt even dream of doing. How many people do you know who have held their breath for even four minutes and finned down to 30m on one breath

On courses run by the Performance Freediving International team (which coached and supported Blaine) you can develop your freediving skills safely to these limits and beyond. Youll learn about the physics of freediving, how your body is affected, technique and emergency drills.

To call your first 30m constant-weight freedive an extreme accomplishment doesnt do it justice - its more a life event.

Casting my mind back over three decades of diving experience confirms to me that extreme is just a label that can be applied to any divers experience.

My first dive in a shallow quarry will always be an extreme moment in my life, as was my introduction to night diving, and the first time I experienced the mind- bending thought that there was more than 100ft of water above me.

I would suggest that its not what other people label as extreme that is important, its what you believe it to be. Its your experience, your magical moment. Perhaps you have your own ideas about what an extreme underwater adventure should be

The Xing Da as she went down
the Jolande (which has been dived down to 205m)
and the Rondo - an extremely
Dead-eye on a perfectly preserved wreck in Canada, the Arabia
mile-high wall-dives such as this one in the Cayman Islands can be awe-inspiring
Dorothea Quarry has a reputation as Britains most challengingdepth-training location
a Spanish galleon complete with treasure - Anguillas El Buen Conseta.
The Atlantis submarine in the Cayman islands can give divers an insight into what deeper waters hold..
Floridas Ginnie Springs, with its remarkable visibility
the stunning Eye of the Needle pinnacle off Saba in the Caribbean
Diving into history in Hawaiis lava tubes
encounters with manta rays
and bull sharks are invariably events to remember
redefining the limits through free-diving