Up ahead, a dark mass looms through the mist As it gains definition, we can see dramatic rock faces and stacks, shrouded in cloud. Spooky. It's Boreray and Stac an Armin, steep masses of eroded volcanic rock rising high above the ocean. We sail past Stac an Armin and Stac Lee - another sinister looking monolith. The dolphins are still with us, as are several gannets -weird, skinny white birds with yellow beaks and black wingtips.

We're passing the northernmost islands in the St Kilda group - a natural UNESCO World Heritage Site, like the Great Barrier Reef, the Grand Canyon and Antarctica. The two stacs are the highest sea stack in Britain - Stac an Armin is 191m high.

We break into open water again, and another landmass appears through the murk. It's the island of Hirta - the largest in the Kilda group. There was never, apparently, a Saint Kilda - the name is thought to be a confusion of the old Norse for 'well'... but there's lots of possibilities. No matter, because the known facts about Kilda are incredible enough. The massive cliffs, overhangs and caves are home to a whopping 37 per cent of the world gannet population.

There are more than 50,000 gannet nests here, more than 60,000 fulmar nests and in excess of 230,000 puffin burrows. And this is on a handful of craggy islands, the largest of which is only a few kilometres across. The islands have their own subspecies of wren - the St Kilda wren, bigger than the mainland species. The great auk - a now extinct flightless bird, used to be seen here up until the 19th century - but the last one was killed, as it was suspected by the islanders to be a witch. The very last one was killed in Iceland in 1844.

There's hundreds of Soay sheep here - a weirdly primitive species, a strange generational throwback, similar to sheep kept by Bronze Age farmers.

There's even a species of long-tailed field-mouse, which is a subspecies of the mainland fieldmouse, and there used to be a St Kilda house mouse too, but it died out after the islanders left in 1930.

The St Kilda Islanders were a hardy bunch, who lived mainly on the flesh of gannets and other seabirds, including puffins, which were a kind of snack food like crisps (potato chips). Famed for their climbing prowess, St Kilda men used horsehair ropes to climb the daunting cliffs, where the collected gannets and eggs. Even their feet developed to aid their climbing - their ankles had uncommon strength and their toe became prehensile. Before marriage, young St Kilda men had to climb to a place called the Mistress Stone, high above the sea, where had to balance one heel while holding his other foot in his hands. This was to prove that he could provide for a family!

In the past, Greenpeace has been active around St Kilda - the ecosystem here was under threat from oil exploration in the Atlantic Frontier area, and we appealed to the UN for sensible action...

Today we sail past, drinking in the dark beauty the caves, arches and rocky stacks. We can see the remains of the deserted village, and the road up to the army radar post on the top of Mullach Mór. As well as the gannets, we had several odd land birds join us - including one bird that may have been the Kilda Wren.

But weirdest of all was Kate's encounter. Out on the bridge deck, she spotted a bird perched on a Greenpeace inflatable. Camera poised, she edged closer... to find a rather petulant looking owl staring at her. She's writing a blog about it - so I'll let her tell the story.

After that, our cruise to these weird islands is over. We shake the rain from our hair, and head below for some lunch. The Esperanza rounds the islands of Dun, Hirta, and Soay, and then sets a course west, while the damp, low-slung sky throws a blanket over St Kilda and its secrets.

- Dave