AS I DESCEND THE LINE the mild, murky surface water goes hazy about 6m down, and for a few seconds the swirls of the thermocline blur my vision.
A little deeper and, bang, the visibility is like gin and the water temperature plummets to a finger-chilling 7°C.
Below me the characteristically green water of Norway’s Trondheimsfjord dissolves to inky black. Ghostly white moon jellyfish pulsate into view, then pass by on the slight current.
My buddy, Norwegian marine-life expert Erling Svenson and one of my reasons for being here, is leading the way. Beneath him lies the other object of my trip to these northern climes: a reef of the coldwater coral Lophelia pertusa.
Coral, reef and Norway are not words you might expect in the same sentence, but this site has become a mecca for those with a passion for temperate marine life.
At around 20m the reef comes into view, a bright white mound on the pitch-dark seabed at 42m. For many divers this depth is routine but it’s around 10m deeper than my usual range so this is a big dive for me, and I’m glad of the week’s fjord diving we’ve already had to work down to it.

I’M ON A TWO-WEEK expedition organised by a team of scientists from Heriot-Watt University in Scotland, the Polish Institute of Oceanography and London’s Natural History Museum.
Between them they are working on four projects, and have dozens of samples to collect for each one.
Dr Joanne Porter and Professor Piotr Kuklinski are two of Europe’s leading experts in bryozoans, colonial creatures commonly known as moss animals.
Jo and her team have been bringing up bryozoan samples all week and every night after dinner the table is cleared, microscopes set up and the specimens analysed and preserved.
Piotr needs bryozoans on rocks, and the bow of the boat is already home to buckets full of small boulders encrusted with the objects of his desire.
Dr Bill Sanderson has two projects but is having less luck. None of the sites we’ve dived in the first week have been home to the horse-mussel beds he and his team have come to study, so hopes are being pinned on the Skarnsundet area north of Trondheim, where we will spend most of the second week.
For Bill’s other project he needs to collect amphipods, but so far his attempts to trap any of these tiny crustaceans have failed.
There has been much discussion over the choice of bait for the traps he sets, and for the Lophelia dives he has been persuaded to try cat-food. Whether or not Norwegian amphipods like pet food won’t be revealed until the traps are retrieved the next day.
Our home for the fortnight is the 21m liveaboard mv Halton, run by commercial diver Bob Anderson. The Heriot-Watt Scientific Dive Team regularly charter the boat, so Bob is used to dealing with the strange demands of marine biologists.
Built in Norway in 1973 for the North Sea fishing fleet, the boat offers living conditions that are rather cosy, but the food is great and there’s plenty of deck-space for kitting up.
The diver-lift makes getting back on board easy, even with the twin-sets, camera systems and survey equipment with which most of us are diving.
My second favourite feature is the Rayburn in the galley – great for cooking casseroles and even better for drying out the odd soggy thermal.
The Halton’s home port is Stromness in the Orkneys and her normal cruising grounds are Scapa Flow, the north coast of Scotland and the Shetlands, but over the past 10 years Bob has been exploring the coast of Norway.
Trondheimsfjord is further north than Bob and the good ship Halton have ventured before and the area is new to the scientists too, so we’re all relying on our onboard Viking, Norwegian underwater photographer Erling Svenson, to find dive-sites that will keep both the scientists and the underwater photographers on the expedition happy.
With almost 40 years’ diving experience under his weightbelt, Erling is hugely knowledgeable on all aspects of marine life and ecology around Norway and beyond. His book Marine Fish & Invertebrates of Northern Europe is now in its sixth edition and the English edition, published in 2003, is a much-used feature of my bookshelf.
Not only are Erling’s images spectacular but the text by his co-author Emil Moen includes masses of fascinating detail about the lives of each species. In recent years I have been admiring Erling’s underwater photos on Facebook and gently picking his brains every now and then on marine life I have photographed myself.
When the chance came to dive with him on this expedition I jumped at it, especially as it included a visit to a Lophelia coral reef, something I’ve long wanted to see.

OUR FIRST WEEK has taken us more than 4000 nautical miles north from Bergen, weaving through the beautiful scenery of fjords, mountains and forests and stopping along the way to dive.
It is just a week after midsummer’s day so, although the sun dips below the horizon, there is no real darkness.
The dives are mostly on sheer walls, with warnings from Bob along the lines of “the bow’s in 8m and the stern’s in 150!”
The first highlight comes in Herdlafjorden on day one. Erling had heard of a spot where spurdogs gather at this time of year. We jump in and, sure enough, dozens of these small sharks up to 1m or more long are gently patrolling the underwater cliff from the shallows to as far down as I can see in the clear, cold water.
Near Florø more inside information puts us on another amazing site, a wall smothered in huge white soft corals, a slow-growing and long-lived species called Capnella that normally occurs so profusely only in much deeper water.
Beyond Florø our voyage north means leaving the shelter of the fjords and heading towards the open sea to round the coast at Stad, also called the Gate, an exposed promontory known for its strong winds and wild seas.
Luck is on our side, however, and with near-perfect sea conditions we decide to jump in beneath the lighthouse that marks the headland, a dive that includes encounters with wolf-fish and anglerfish.
Exploring new areas also reveals one or two sites that are a bit less dramatic, but the excitement of diving where few or no divers had ever been before far outweighs the occasional disappointment, and with little experience in northern climes I always find something I haven’t seen before.

FOR ERLING TOO there are new discoveries, despite the thousands of dives he has done in Norwegian waters.
On a dive at Munkholmen, a small island near Trondheim, I notice his strobes firing 10 to the dozen, a sure sign that he has seen something exciting. He is photographing a Norwegian red fish, a species that to me looks like one we have seen on most dives, but on Erling’s insistence I take a few shots myself.
Having plumped for my macro lens on this dive I can capture only its portrait. As soon as we hit the surface Erling explains that what we’d seen was in fact Sebastes marinus, a deepwater red fish that has never before been photographed in the wild.
Back on the boat he enthuses over his find, proud to be the first person in the world to capture the fish on film and that I am the second.
At Trondheim it is time for a day off and a chance to explore the historic city and purge our systems of nitrogen before the deeper diving to come.
The next morning we steam north to the Tautra Ridge, home to the shallowest-known Lophelia coral reef in the world and our dive-site for the next two days. Once near the seabed my eyes become accustomed to the gloom and the brilliant white and pale pink Lophelia seem to glow in the darkness.
At just 4m high this network of fragile, intricately branched coral is a baby compared to those found in deeper water that have been measured at more than 30km long and 35m high.
These huge living structures are found hundreds of metres below the surface of the north-eastern Atlantic, particularly around the continental shelf off Ireland and Scotland and the Sula Ridge off Norway, and are thought to be thousands if not millions of years old.
Sadly, as quickly as the extent and biological importance of these reefs are being discovered the threats are increasing, with many areas already terminally damaged by the deepwater fishing and oil-exploration industries.
Lophelia creates a rich habitat for hundreds of other species, and as I swim around the reef I see fish, crabs, sponges, fan corals and basketstars living among its branches. With a growth rate as low as 4mm a year, even the small reef in front of me could be more than 1000 years old.
As Erling explains, diving here provides a chance to peek through a window on the world of the ocean depths, a place normally viewed only from a submersible. Seeing it with my own eyes is a real privilege, and an experience I shall never forget.

WITH THE DIVES on the Lophelia completed, Bill has retrieved his amphipod traps and we’re all eager to see what may have succumbed to the lure of the cat-food. Inside there is just a single amphipod, scientific proof that either few amphipods live on Lophelia or that amphipods don’t like Kitekat!
Steaming further north we arrive in the narrow fjord of Skarnsundet, our base for the last few days of the expedition. Time is running out for the horse-mussel team, but close to the tiny hamlet of Venneshamn they finally hit gold, with large areas of seabed covered in their favourite mollusc.
Samples are retrieved and the biologists spend hours happily measuring, dissecting and pickling the enormous mussels. Sadly for the rest of us, horse mussels are no good to eat!
The diving in Skarnsundet is as good as Erling promised it would be, with more sheer walls and spectacular marine life. Huge, bright orange Paramuricea fan corals thrive in the deeper areas, with colourful soft corals, starfish, urchins and cucumbers competing for space in the shallows.
There are also literally hundreds of nudibranchs, much to the delight of the photographers, and the spiky red stone crabs that often perch beneath the big pink Arctic anemones are also popular among those of us with cameras.
The stars of the show here, however, are the helmet jellyfish Periphylla periphylla, deep red creatures that normally inhabit very deep water and are shaped like 1950s Russian spaceships.
A combination of tide and topography means that they are common here, and watching them waft by in the clear green water as I count down the minutes on the safety stop of my final dive is the perfect way to end my Norwegian adventure.