Dan Burton's rapid, staccato voice enthused me through the decompression schedule: We started our stops at 50m, still using our bottom mix. Robb Mooney, our safety diver, waited at 40m, wearing a full-face mask with surface comms. After he had told the surface that we were okay, we ascended to 33m, where we switched to nitrox 36. We then switched to a surface demand system which supplied us with a 50/50 nitrox mix at 18m. Our final (and longest) stops were at 6m and 3m on 100 per cent oxygen - extremely boring at 3m I can tell you.

The shallow stops may have been boring, but what Dan and buddy Billy Deans had found on the bottom at 92m most certainly was not: half a million silver coins, minted in 1783 - around 50 millions worth at todays prices!

El Cazador had disappeared on a voyage from Veracruz, Mexico, to New Orleans in January 1784. The purpose of her voyage had been to take hard currency to shore up the crumbling economy of the land in North America controlled by Spain at that time - a million square miles between the Mississippi River and the Rocky Mountains called Louisiana.

Nearly two centuries later, in August 1993, a fishing boat, Mistake, fouled its nets 50 miles off the coast of Florida in the Gulf of Mexico, on what was believed to be a flat, silty bottom. On hauling the severely damaged nets in, the crew discovered rocks in them. A closer examination revealed the rocks to be clusters of silver coins adorned with the face of Charles III of Spain, and dated 1783.

Nautical archaeologists revealed that the ship was very probably El Cazador, and that its cargo was very valuable. A partnership was formed to salvage the kitty, and Mistake returned to the spot, and lowered a video camera. After a two-day search, a dramatic image came up on the camera: a mass of coins, 4-5m wide, 10m long, and up to 30cm thick. Using a saturation dive team to recover the coins was costed at $10,000 per day. Instead, the salvage company, Marex International, opted to try technical divers for the job - something unprecedented in the field of salvage.

Marex contacted well-known technical diver Billy Deans, owner of Key West Diver & Deep Sea Technologies. One of the divers chosen by Billy Deans to take part in the 1994 salvage was 29-year-old Brit Dan Burton, from Chichester, Sussex. Dan takes up the story:

Obviously, cost-wise there are huge advantages in using technical divers. They can work more quickly than a commercial diver with a surface tether - and at far less cost. The disadvantage is, obviously, that they cant stay down there all day and then decompress in a bell.

Nonetheless, the six-man team was getting a fair bit of bottom time. We were organised so that three teams were diving twice a day in relay with between 20 and 30 minutes bottom time and just over three hours decompression, explained Dan.

The entire team was aware that a salvage of this nature had never been carried out using technical divers before. We knew that everybody was watching to see how it was going. It was a ground-breaking operation, said Dan.

After diving with Plymouth Polytechnic SAC, Dan qualified as a Dive Master with NAUI in Taiwan. He is an IANTD Nitrox Diver and a IANTD Trimix Dive Supervisor.

It has been just over five years since Dan first stumbled across the world of technical diving when he offered to fill bottles for Key West Divers during summer holidays, while still a college student. On the Cazador expedition, he was given the responsibility of gas mixing. Because all eyes in the salvage and technical diving community were on the team, safety margins were uppermost in his mind. We did everything by the book, explained Dan. Safety is obviously crucial with this type of diving operation. I mean, the boat was 50 miles offshore! We didnt push any limits; just down the line, do the job and up the line again.

In the event of one of the divers having a problem, the salvage boat, RV Beacon, carried a US Coastguard-certified double lock recompression chamber, with a qualified diver medic technician in attendance. It didnt get to see any action though.

One of the main features of technical diving is backup equipment which can be resorted to in case any part of the kit fails to function. Backup is crucial, echoed Dan as he outlined the measures taken by Billy Deans team.

I had two sets of submersible tables for each of the three gas mixes. A couple of computers. I always take a backup timing device as well - you never know, under pressure glass can sometimes implode and you wouldnt have a clue how long youd been down. It happened to Sheck Exley (the world-famous cave diver who died in 1994) once; he had to count his decompression! Food for thought: 150 minutes of decompression makes for a lot of fingers and toes!

Did he have to resort to any of his backups Actually, no. But its better to plan for the eventuality, wouldnt you say

Initially, though, the team had to request fresh tables from decompression expert Dr Bill Hamilton. The ROV had reported that the wreck was at 83m but, having brought out repetitive tables for that depth on their 2-day voyage to the site, the team discovered the wreck was actually at 92m! With modern technology, though, it didnt delay the start of the diving for too long. After a quick call from Billy Deans, Dr Hamilton recut the tables and sent them to the salvage boat by satellite fax - all within 10 minutes.

As the team was intending to spend 30-40 days offshore, bringing all their gas pre-mixed was out of the question. A total of 139 bottles were therefore loaded onto the Beacon at the start of the voyage. These included: 40 bottles of helium, 50 bottles of oxygen, and 12 sets of double tanks. A portable gas mixing panel was used to blend the gases aboard. We were diving with twin 18-litre low-pressure tanks, Dan explained. You fill these to around 180 bar.

The team werent wailing for a good fill like so many recreational divers then Well, trying to fill a tank to 300 bar is, quite frankly, a pain in the arse. Its easier to fill a tank to 170 bar. I certainly wasnt filling anything to more than 200 bar. Again, its about not pushing limits. The key things when youre pumping oxygen under pressure is to keep it cool, pump it slowly, and make sure everything in the area is clean; no oil, particularly.

Twin 18-litre tanks Yeah, they weigh around 200lbs - theyre a real hernia set! But obviously, once youre in the water, you dont feel the weight.

I put it to Dan that it might have been easier to use a rebreather. Not only would you not have to lug twin 18s around, but you could also control your gas mixes.

We did think about it, but the problem was that there really wasnt anything to use at that time. I mean, there were units available, but nothing that was user-friendly. There are more around now.

So how did he find the diving in the Gulf of Mexico It was really clear until we reached about 75m. But then the conditions changed rapidly to a sort of chocolate-brown soup. I couldnt see the bottom until I hit it! said Dan. To quote Billy Deans, the bottom was like oil half a million years before it became oil.

Dan continues: It was pitch black. I couldnt see Billy, but could hear him breathing. We both had 50 watt lights, but they couldnt penetrate the silt. Feeling his way over the seabed during their first dive, Dan came across some round objects in the silt. Grabbing a handful, he reached along his buddy line to Billys arm. With the poor state of the viz it was impossible to see what the objects were, so the pair ascended 5m and shone their lights on them.

We just yelled with excitement, said Dan. Dan and Billy used the rest of their bottom time to load up a bag with around 35kg of coins, using a lifting bag to send it to the surface.

A couple of days later, the ROV relayed pictures of a large, green object sticking out of the mud on a night survey.

The following day, diving in the secondpair, Dan ran his reel out 12m from the ROV and discovered, half-concealed by the mud, the El Cazadors bronze bell. Tying off a line to it, he greeted Robb Mooney at 52m with a diagram of a bell on his slate. Mooneys eyes lit up. The bell was recovered using a cradle on a tendered line.

In a planned technical operation like this, the dives themselves were highly disciplined. Every aspect of the dive is planned: descent, bottom time, everything. Once its time for your first stop, you go up and do it, no hanging about. Its much more regimented than the way people normally dive. The guys on the surface know that if you say youll be back after 210 minutes, youll be there within a minute. Another plus is that you can work out your gas consumption rates far more accurately.

Dans favourite time to dive was at night because the viz was at its best. Its kind of fun diving at night to 90m, said Dan, particularly if the current gets up, because it blows the silt away.

Night dives were made slightly more exciting by the thought of what was swimming in the Gulf. Its full of sharks. Theres not much you can do though... bring your shark repellent ybe! I didnt come across any on the wreck - just mantas and small fish - but three of us were diving to free up a friends shrimp nets and a big bull shark came straight for me. When I looked around, I saw that the place was crawling with them.

A regular photographic contributor to aquaCorps - the American technical diving magazine - part of Dans role on the Cazador job was as expedition photographer. Getting the coins up was definitely the number one priority though. It wasnt easy to take photographs. Every day Id set up my camera gear and theyd say Youre not taking that down. Then at the end theyd say Where are the photos and Id say You didnt let me take any!

The Cazador team made 22 dives and recovered approximately 12,000 coins during a ten-day operation. The project had attracted the attention of the US Coastguard, however, which decided that commercial diving safety regulations were not being fully adhered to and shut the salvage down.

Now based back in Britain, Dan has recently finished a masters degree in photography at Plymouth College of Art and Design. He is clearly excited by the possibilities of using technical diving methods to do work formerly reserved for divers equipped with a surface tether. Last summer, Americas National Oceanic and Atmospheric Administration (NOAA) asked Billy Deans if his team would demonstrate their working set-up on the USS Monitor off the coast of North Carolina.

The Monitor was the worlds first iron-clad battleship. She now lies at 70m. Its a strenuous dive: The current rips through there, stated Dan bluntly.

There is talk of returning to the Monitor to carry out an archaeological survey of the wreck this summer. Plans have yet to be finalised, but Dan would like to return as part of the team. In the meantime, though, hes back off to America to photograph NASAs underwater astronaut-training facilities.