I made my way up on to the deck just as the sun broke above the horizon. To the west was Village Bay on Hirta, St Kilda’s largest island bathed in glorious morning light. To the north the imposing cliffs of Boreray and the iconic pillars of Stac Lee and Stac an Armin rose out the sea like primeval sentinels keeping watch over Hirta. As I took in this dramatic panoramic vista of the UK’s most remote archipelago, I felt a renewed vigour. We had made it at last.

The waters around St Kilda are designated as a Special Area of Conservation (SAC) for their rocky reefs and sea caves, which host a wealth of spectacular sea life. We were part of a scientific dive team commissioned by Scottish Natural Heritage (SNH) and led by Heriot-Watt University to undertake Site Condition Monitoring around both St Kilda and North Rona. The purpose of Site Condition Monitoring is to establish whether features of SACs are likely to maintain themselves in the medium to long term under the current management regime and wider environmental influences.

We had three weeks to complete the survey. This sounded like plenty of time but we were all aware of the potential difficulties in getting out to these remote islands due to strong winds and large swell, as well as all the possible complications that diving inside caves can present. We would reach St Kilda on our survey vessel, the M/V Halton, a rugged 21-metre converted trawler skippered and owned by Bob Anderson, a seasoned and experienced operator and diver. If anyone could get us there, Bob could. But, as we set off from Stromness, the forecast didn’t look good.

Heading down the dramatic west coast of Hoy to mainland Scotland, we aimed to get as far west as possible on the first day. As we progressed in a moderately uncomfortable swell it quickly became apparent from incoming weather reports that heading straight to St Kilda might not be sensible.

Contingency plans for bad weather were put into action by Dr Dan Harries (Heriot Watt lead scientist) and Dr Lisa Kamphausen (SNH marine biologist). We headed to Loch Eriboll to refine survey methods and logistics.

Jutting inland for almost ten miles, Loch Eriboll is the largest sea loch along the north coast of Scotland. The rugged coastline supports numerous sea caves visible at the base of steep cliffs, many of which remain unexplored.

The shelter of the loch gave the team a perfect opportunity to get organised and put some of the survey methods into action. The most time-consuming part of each survey was actually finding a cave which could be dived. We were dependent on the amount of swell, tide and resulting surge – even a small amount of swell at the wrong point of the tide caused a surprising amount of surge inside the caves making surveying incredibly difficult.

We surveyed two caves in Loch Eriboll. Both began as large cracks in the cliff wall forming deep gullies. The gullies narrowed and ascended leading on to the back, each at around 100m. Once a little way inside the caves, the sunlight quickly faded. The water here was beautifully clear and, turning around to look back gave spectacular views out to the entrance, steep walls rising up to shimmering, iridescent blue-green ripples on the surface.

Following the success at Loch Eriboll we pressed on enthusiastically, still cautiously optimistic that we would be getting to St Kilda by the end of the week. However, the forecast remained less than ideal with those low pressure systems still looming over the Atlantic. In the meantime, we steamed around Cape Wrath and started down the west coast of Scotland, heading to the next contingency location, Loch Laxford.

Less of a distinctive sea loch than Loch Eriboll, Loch Laxford is more a series of rocky inlets, small islands and waterways. This remote section of the northwest coast of Scotland has a fascinating and slightly other-worldly landscape. Its sparse, seemingly inhospitable rocky peaks and outcrops give a distinctive feeling of isolation which is quite rare in the UK.

Weather reports continued to come in and the news was increasingly dire. Whatever hopes remained of getting to St Kilda by the end of the week were promptly washed away by reports of an 18ft swell, kicked up by winds far out in the Atlantic. It gradually became clear that St Kilda was going to elude us, certainly for the next week, so the decision was made to shift the focus of the survey, taking advantage of the shelter offered by Loch Laxford.

Rocky reefs are a designated habitat of the Loch Laxford SAC and, as such, are part of the Site Condition Monitoring program. They can be incredibly diverse habitats, providing shelter to species such as corals, sponges, sea squirts, fish, crabs and lobsters.

The basic approach of the rocky reefs surveys involved diving along a transect line, recording all marine life, estimating abundance, taking still photographs and video as well as collecting specimens. The actual transect started on the shore where another team conducted an intertidal survey. Additional surveys from rocky sites that were not so easy to dive were obtained through the use of a drop-down video system operated from the main vessel.

The survey sites were an interesting mix of shallow, kelp-dominated communities and slightly deeper boulder habitats. A couple of the sites consisted of some fairly extensive maerl beds, a fascinating habitat made up of calcareous red seaweed which grows as unattached rounded nodules on the seabed. Maerl beds are an important habitat for numerous types of marine life which live amongst or attached to the surface of the maerl, or burrow in the coarse gravel below the top living layer.

The week turned into the weekend and, after seven days at Loch Laxford, we still weren’t any closer to our distant goals. However, there was a glimmer of hope – a small area of high pressure was creeping north which could potentially open a window of opportunity. Spirits cautiously raised, we pressed on westward to the Hebrides ready for the dash out to St Kilda.

It made sense to initially aim for Loch nam Madadh on North Uist, another Special Area of Conservation for its rocky reefs. Here we could undertake survey work in the shelter of the sea loch but still be well-positioned for the trip to St Kilda if the chance arose.

The first transects began a little way out from a rocky island, just at the entrance to Loch nam Madadh, on a boulder field at a depth of around 20m. Boulders often form fascinating habitats, their relatively flat surfaces providing ideal platforms for numerous species to colonise. The boulders here were covered in an impressive array of large, iridescent Devonshire cup corals which were dotted on almost every surface. There were also frequent dense clumps of dead men’s fingers and plumose anemones, numerous hydroids and sea squirts, vibrant cushion stars, and also good numbers of the exquisite soft coral Swiftia pallida, the northern sea fan, only found at a few sites off the west coast of Scotland and southwest Ireland. There was no shortage of fish, and for most of the first transect we were continually circled by a large school of pollack, always just on the edge of view. We also had the good fortune to stumble across a couple of adult anglerfish, well camouflaged, cruising just above the sand.

The boulders turned to steep walls which ran up to the surface. At around 10-15m, where the sunlight had penetrated the clear blue water, a dense kelp forest had formed, the large, fan-like fronds drifting back and forth in the surge. Peering between the thick kelp stems revealed an abundance of multi-coloured jewel anemones and mats of bright pink encrusting algae.

At the end of the second day in Loch nam Madadh, the Atlantic swell dropped. With five days left it was now or never. Steaming through the spectacular Sound of Harris we headed west into the setting sun, towards St Kilda.

There can be few places on the UK dive map with a reputation quite like St Kilda. Remote and, as we discovered, often inaccessible, the distant archipelago has attained an iconic status among UK divers, its name often spoken with reverence by the lucky few who have had the opportunity to visit. It is the most remote part of the British Isles, made up of several islands and sea stacks created by ancient volcanic activity which dominated the west coast of Scotland over 60 million years ago. Its isolation from mainland UK and any significant sources of pollution, run-off and sedimentation results in exceptionally clear, blue oceanic water. This, combined with its dramatic underwater topography, provide conditions which support a level of marine flora and fauna not commonly seen around the British Isles.

Adding to its mystique, St Kilda also has a fascinating human history. Despite being isolated and perpetually battered by hostile Atlantic conditions, people thrived here for centuries until relatively recently, surviving largely by scaling sheer cliff faces in order to catch the plentiful seabirds. Unfortunately, in 1930 numerous factors led to the evacuation of what remained of the population.

In 1987 St Kilda received dual World Heritage status, making it one of only 24 global locations to be recognised for both its natural and cultural significance. The archipelago remains crucially important for wildlife and hosts huge populations of seabirds, including the world’s second largest colony of northern gannets.

We arrived in the early hours, just as the sun rose over Village Bay. A wave of enthusiasm and motivation swept around the boat as everyone appeared on deck. Remarkably the weather had cleared completely and, as the sun came up, we all gazed up at the blue sky above the steep cliffs of Hirta to see clouds of puffins circling overhead. Conditions were about as good as we could hope – it was time to survey the sea caves of St Kilda.

The scouting team headed off in the zodiac and, after an hour or so, returned having identified a number of potential sites. By now the survey routine had been polished to military precision so we were kitted up, on the zodiac and in the water in no time at all. The visibility was predictably very good and the marine life was striking enough to make you realise you were diving somewhere quite unlike anywhere else in the UK. On a simplistic level it is the vivid colours that provide the most characteristic feature of diving around St Kilda. Intense pinks, greens and oranges on a rich turquoise canvas.

We surveyed three full cave systems. The cave entrances were marked by dense kelp forests and carpets of iridescent jewel anemones, literally hundreds of thousands packed onto the cliff faces and boulders. This changed from kelp to animal turfs as the light became less the further into the cave you went. The caves tended to narrow as we got further inside, looking up our trapped air bubbles looked like liquid metal as they danced across the cave ceiling. Most of the caves ascended toward the rear. Turning around and swimming slowly back toward the cave entrances always provided a spectacular sight, a window looking out into the open ocean with the rays of light dancing through the clear, blue water.

Our final dive on St Kilda was a rocky reef survey underneath the natural archway on Dùn. Separated from Hirta by a narrow channel, Dùn provides Village Bay with protection from prevailing south westerlies. Essentially a narrow, vertical crack in the island the archway can be subject to some fierce tidal surges. This results in some spectacular marine life all placed to take advantage of the never-ending food supply made available in the passing currents. Jewel anemones were so numerous the underwater cliff faces looked like giant rainbows. The walls at the entrance were covered with dense thickets of hydroids, and even the hydroids themselves provided shelter for what must have been millions of tiny mysid shrimps.

Although similar underwater environments can be found elsewhere in the UK, there is something special about St Kilda. Perhaps it is simply the knowledge you are diving in a remote place on the very edge of Europe. Rugged, wild and untamed.