LIKE MANY DIVES, IT DOESNT LOOK like much from the surface; just a ring of floats among many such rings of floats moored a few miles off the north coast of Malta.
However, even on a slightly choppy sea, I can see that something unusual is happening inside the ring. The water looks rougher, and more confused.
Mike Busitil from St Andrews Divers Cove ties the boat up to a buoy just outside the floats, and Emi Farrugia, project manager for the Marine Adventure Park, takes over with a briefing for the dive. He then swims to the ring of solid floats that supports the tuna pen, and climbs on top to help divers into the enclosure.
In the water, all I can see from outside the pen is the heavy net hanging from the ring of floats. I hand my camera to Emi, then push up and do my best to protect the gauges of my rebreather from dragging, while he and one of his colleagues pull me over the float and into the pen. Others soon follow, some in scuba kit, some snorkelling.
As I descend, a grey blur soon becomes an unending wall of silver, flowing past at a constant 4 knots, stretching from just beneath the surface to as far below as I can see.
Are there more bluefin tuna than water inside the net If the whole lot was separated and put into two buckets, would the tuna tip the scales
I swim towards the tuna, and the silver wall parts to flow around me. Its like being a hedgehog on the motorway, relatively safe on a white line, but petrified that some driver will change lanes at the wrong moment.
However, none of the tuna do. Their shoaling reflexes are so finely tuned that they never bump into each other, or any of the divers.

EXCUSE ME WHILE I DIGRESS onto one of the better-known organised big-fish encounters - the shark feed.
In the Caribbean, the main sharks involved are Caribbean reef sharks. These grow up to 3m long, with a maximum weight of 70kg. At a typical feed there may be anything from 10 to 50 sharks - up to 3.5 tonnes of fish.
Compare this to the average adult bluefin tuna, which is 2.25m long and weighs 350kg, though it can grow up to 4.3m long and 680kg. Even the smallest is much bigger than me, and there are about 1000 of them in the pen. Thats about 350 tonnes of fish, or 100 times as much fish as on a big shark feed.
Whatever the statistics, the experience is way more than mere numbers could prepare me for.
I am simply overwhelmed. For the first 15 minutes, I dont know where to turn, or which way to point my camera.
As I move further into the shoal, I find myself descending, drawn into the whirlpool created as the fish circle the pen. I break through into the centre and find myself in a mass of bubbles. I look down to see who is below me, but no one is there. Then I look up to see my buddy Dino above, and it is his bubbles being sucked down by the vortex.
Buoyancy control in the vortex is a bit difficult. I use my wing BC to over-compensate, combined with swimming upwards, then dump rapidly if I stray out of the strongest downcurrent.
Its a good job the enclosure has a net floor. As I drift deeper, I end up standing on the base of the net at 35m. Have you ever been beneath a spiral of jacks or barracuda Its like that, but on a much bigger scale.
Buoyancy control is easier in the deeper water, but this deep the sheer mass of tuna cuts out much of the available light, so I put some effort into staying somewhere between 15 and 20m, in the middle of the spiral both horizontally and vertically.
Emi has given small pots of fish to some of the divers, with instructions to throw out the sardines one at a time. The tuna will not hand-feed, and need room to swallow them whole without halting their relentless swim.
Depending on its size, a tuna will need to be fed between 9 and 20kg of smaller fish to gain 1kg in weight, so the amount handed out by divers is trivial compared to the overall mass of sardines, mackerel and squid dropped into the pen at feeding time.
The disparate size of the predator and prey, the distance they maintain and
the speed with which they swallow the sardines makes it an event that is hard to see by eye, let alone catch on camera. It is just too small to show up.
I give up on the feeding, and spend the next 10 minutes catching Dino in the middle of the spiral, before again turning my attention to individual fish.
To try to slow things down, I get myself vertical in the water and spin anti-clockwise, in the same direction as the tuna. This keeps an individual tuna in my field of view for longer, but they are still overtaking me as they endlessly circle the pen at 4 knots and, an unanticipated side effect, because I am looking at an individual for longer that individual grows wary and edges away.
The more easily anticipated side-effect is that I soon get dizzy.
I pause to reconsider, then try spinning the other way, against the flow of fish. Now they flash by as relative motion is speeded up, but I do catch them that little bit closer. I still get dizzy, so change tactics again.

I MOVE HALFWAY TOWARDS THE EDGE of the pen, and start swimming against the flow - back on the motorway, but now heading into the traffic rather than across it. It seems that every tuna in existence is about to run me down, but this is just a trick of perspective, and they scream past to either side.
Visibility is deteriorating. Having just been fed, the source of the fog becomes obvious as an intact lump falls past me. I am in the middle of a cloud of tuna poo. Now is not the time to remove a regulator or rebreather mouthpiece.
My solution is to head back to the outside of the spiral, and go shallower. Fewer tuna above me means less poo falling past, and the visibility is considerably better.
I come to the end of the dive, having filled many gigabytes of memory card. The others are all back on the boat, waiting for Dino and me to finish.
Like many big events, after an hour I am in sensory overload and immune to the spectacle. Would I do it again Certainly. Would I do it tomorrow Probably not, I like varied diving, but perhaps in a few days I will be ready for another visit to the tuna maelstrom.

Diving with tuna is managed by the Marine Adventure Park (www.marine, but divers can book through dive centres such as St Andrews Divers Cove, Gozo ( Other park pens contain rays and barracuda.

... have the latin name Thunnus thynnus
... are found in sub-tropical regions of the Atlantic, Mediterranean and Black Sea, with distinct populations
... return to spawn where they were born - in the Mediterranean or Caribbean
... lay fertilised eggs that hatch after two days The baby tuna start to feed immediately, and reach 1m long and 40kg in the first two years.
... average 2.25m long and 350kg as adults The record catch off Nova Scotia was 4.3m long and weighed 680kg.
... are sexually mature after 4-5 years
... as females produce 10-40 million eggs a year
... live for 15 to 30 years
... dive as deep as 985m
... come close to the surface to spawn, which in the Mediterranean occurs between May and July
... cruise at 4 knots, but can swim at 20 knots or more when hunting
... are capable of crossing the Atlantic in 50 days
... fear as natural predators great white sharks and orcas
... can fetch 100 euros a kilo in the Tokyo fish market
... are claimed by some to be fished at four times the sustainable rate and are on the ICUN Red List of threatened species