Black water, open ocean

THE SHOOT: Black-water diving, and open-ocean critters
LOCATION: Off Hawaii
CREW: 1x director, 1x cameraman, Richard Wollocombe; 3 x divemasters
SETUP: Dayboat, scuba/breath-hold

Parts of the Pacific hold the clearest waters on the planet, clearer than freshwater lakes and, even more incredibly, clearer than the water in your tap. With few nutrients in these waters to feed the food chain, marine scientists often refer to them as marine deserts, because there is little to sustain life.
So, why would a BBC team of divers and film-makers head out into open water in the middle of nowhere, in the largest ocean on Earth?
Well, for once it's not the big and bitey we're after - this time it's the small and the unseen.
Hidden in this seemingly empty ocean is a surprising and fascinating world of bizarre and often overlooked life-forms, where adaptations to low-energy living have resulted in unearthly, sci-fi style creatures.
What will be the third film in the South Pacific series, Endless Blue, is about the challenges of living in the vast void that is the open ocean.
These surface waters may look barren by day, but every night there is a vertical migration, as life-forms that shun the light of day rise from the deep to predate on a micro scale in shallow waters.
Our plan is to dive the same waters day and night, revealing the changing face of the ocean, and the mysterious and normally unseen shift in the communities.
First, we head out to look at the daytime communities. We have brought along Hawaii marine scientist Dr Andrew West, who is an expert on all things planktonic, and specialises in finding tiny fish larvae in seemingly empty stretches of water.
The trick, Andrew reveals, is to look for "current lines" on the water's surface. These are those flat tracks that you can see criss-crossing the ocean's surface on the clearest, flat-calm days, looking like rivers in the sea. Caused by pressure waves and swells, these tracks are formed where water bodies converge, bringing with them all manner of floating objects and drifting life.
In this ocean environment shelter is hard to find, and any floating object can serve as a refuge for fish and other organisms.
The sargassum frogfish has taken this to a whole new level, and it is top of our wish-list of filming subjects. It lives out its entire life at sea, floating wherever the ocean takes it, on objects that drift on the currents.
In our small dive-boat we comb the ocean surface, moving from track to track in search of the flotsam and jetsam that might signal life beneath.
It's soon obvious that these tracks are distinct patches in the ocean, and as we cross onto one, we start to see tiny specks floating in the water.
Weird planktonic shapes drift by, and in patches the water is filled with thousands upon thousands of small red-brown balls just a couple of millimetres in diameter - fish eggs.
Slowly cruising down these oceanic paths, we look out for drifting objects. A piece of net floats by below the surface. It's just a couple of metres wide, but we jump in to check it out. Sure enough, there are a number of small brown drift fish hanging around it, using it as shelter.
The net doesn't look too old, but already each rope has a fine coating of green algal filaments.
Large stalked barnacles have also taken hold, and there are even small crabs and shrimp on this mini floating eco-system.
It's fascinating to think that this small net is the entire basis for these creatures' lives, an entire world floating in the oceanic void.

RETURNING TO THE BOAT, we comb the current lines for other objects. Andrew's eyes are fine-tuned to picking out microscopic organisms, and he's able to spot tiny 2cm larval swordfish swimming right at the water's surface, a perfect silvery miniature, with its giant sword-mouth taking up half its tiny length.
We also find the bizarre pelagic nudibranch, Glaucus. Looking more like an inflatable bath toy than a real living creature, it spends its entire life floating at the ocean surface, feasting on tiny bluebottle jellyfish and other medusas.
As we go on, we see more and more surprising finds, but it's not until a few days in that we find the frogfish. Camouflaged on a rope, in true frogfish style he does very little, waiting for food to swim within reach. But what he lacks in animation, he makes up for in character.
Avoiding swimming at all costs, he prefers a blundering moonwalk to get around his rope, and his grumpy expression is alleviated only when an unwary shrimp swims within range - his mouth springs open, gulping it in whole.
Frogfish are voracious predators, and will even eat each other if they get the chance.
In this empty ocean, literally any object attracts life. A giant floating plank of wood encrusted with barnacles hosts schools of oceanic triggerfish, and stunning mahi mahi. Even a bunch of sad-looking crumpled helium balloons, presumably lost by a child, harbour a school of fish beneath.
But between the current lines and these chunks of debris the water is clear, and little life can be seen. The endless blue ocean stretches off emptily in the heat of the day, and the light rays dance and sparkle in the intense blue oceanic water.
The night dive is a very different scenario. Feeling far from the bleached-out heat of the day, we prepare for a long night ahead.
To give our deepwater subjects maximum time to get to the shallows, we really want to be out on the water around midnight or so.
Fuelled with copious amounts of black coffee, we jump into the boat at around 11 and head out to sea. It's a dark night, and as the lights from the shore diminish into the distance, there is nothing but us, the ocean and the stars.
Eventually we find our spot, and crack out more coffee as we get kitted up. I'm layering the wetsuits, prepared for inactivity, and the lack of sun to warm us. Unlike on most night dives, we are nowhere near a reef or wall. We drop
a sink-line as an underwater reference point because, with little natural light from above, our only other cues to orientation will be our bubbles and the distant lights of the boat.
Once the filming starts, our eyes will be dazzled by our own lights, and with no external subliminal cues to orientate by, it will be all too easy to lose our position.
With this in mind, I have allocated one diver to keep a tight watch on everyone's depth and make sure we remain in visual contact with the boat.
We also have cylumes on tanks, and torches and whistles for signalling at the surface, should we get separated.

DROPPING IN, IT'S A DIFFERENT WORLD. The water feels warm at first, and we orientate to each other in the blackness, and head down to see what's around.
The dark waters are always somewhat ominous. Our torches are beacons, loudly announcing our presence to the creatures of the night, but our senses are stunted, restricted to a few feet of vision in one direction at a time.
Two thousand metres of black water lie below us, and thinking back to the tiger sharks and oceanics of our previous shoots in this region, I hope it's not the time for another close encounter.
At night the water seems thicker, and the torchlight illuminates countless suspended particles, hanging stationary around us. In the absence of other cues, I use these to gauge my movement and position. They appear to sink with each inward breath, and rise on exhalation.
We each hunt for critters of interest in the darkness. The first thing we find is a red siphonophore - a colonial chain organism with an intricate network of stinging feeding tentacles. It hangs in the black with tentacles extended, occasionally reeling them in to feed.

OVER TIME, WE SEE MORE and more bizarre animals. Larvae of crabs and mantis shrimps are transparent ghosts of their adult forms, and giant 30cm comb jellies glide into view, their iridescent cilia hairs refracting our lights into pulsing rainbow stripes.
Ever-curious squid flit in and out of our light-beams, and small glowing pyrosomes make tracks through the plankton soup.
Many animals are flattened to increase surface area and drag, so less swimming is needed to avoid sinking. A slipper lobster larva has taken this to extremes - its bizarre, over-sized head is totally see-through and, despite being around 3cm across, flattened to only a millimetre thick.
Two goggling stalked eyes protrude from its strangely constructed head, and in its pincers and legs it somehow holds four small jellyfish, the size of jelly sweets.
Apart from occasional blunders with each other, these animals will never encounter a solid surface. Living deep and away from the surface currents, this is one of the most constant environments on Earth, and the animals are free to evolve the most delicate of structures. It's a dark world of the unfamiliar and seemingly impossible.

The six-part series South Pacific begins on Sunday 17 May on BBC2, at 9pm