(CNN) — It's mid-morning when three velvet-helmeted swimmers suddenly bob up from deep underneath the turquoise sea.
Dazzles of light dance as kayakers swoosh oars through glacier-clear water, propelling over a kelp forest towards Nor Nour, a tiny, deserted island.
It's then the divers appear: Atlantic Grey seals just meters away, rare in many places but not here on the Isles of Scilly, an isolated archipelago off the UK's Cornish coast, slung out into the middle of the Atlantic.
Nornour is part of the Isles of Scilly's Eastern Isles. It's about 25 minutes of paddling from St. Martin's, one of five inhabited islands.
As usual, Nornour's moon-white shores are deserted, as is the prehistoric village which survives there.
The village has grass-lined circles of stone: remnants of Iron Age huts, still standing despite having been lived in more than 2,000 years ago.
Previously hidden beneath the sands, the houses were revealed In 1962 -- along with more than 300 enameled Roman brooches and Gallic clay figurines -- by one of the storms which sometimes blitz this distant place.
It's believed to have been the site of a sea goddess shrine, where Roman traders once stopped.
Kayaks to reach Nornour can be rented from St. Martin's Watersports, a small company owned by Anna Browne, who made the island her home after visiting on a dive holiday and becoming instantly smitten.
Now, entrenched in island life, Browne wild swims regularly, is on the committee of the islands' newly built star-gazing observatory and runs the local book club.
The attraction is understandable: these islands are like a chunk of the Caribbean in Cornwall.
Protected by the Gulf Stream they (almost) never get cold so exotic plants, unseen elsewhere in Britain -- and brought here by Victorian plant collectors -- flourish. Below the waves, the archipelago is encircled by an underwater garden.
"Underneath our ocean is an array of life, different to warm-water reefs, but just as colorful," Browne says. "We have jewel anemones in pinks, blues, oranges, yellows and greens; sea fans, cat sharks and rainbow wrasse. And you can dive among famous shipwrecks like the Cita and Schiller."
Islands for all seasons
St. Martin's -- one of five inhabited islands in the Scillies.
Matt Cardy/Getty Images
Back at the white sands of Par Beach, where Browne operates, she tells CNN Travel how once she spied a Bowhead whale -- usually an Arctic species -- only meters from the spot she's standing on. "You never know what you'll see here," she says enthusiastically. "At the moment we have hoopoes and bee-eaters." There's a variety of places to stay on St. Martin's, including its only hotel, Karma St. Martin's. It's also possible to rent a cottage on Churchtown flower farm; or from Holly Robbins and James Faulconbridge, a young couple that recently moved over from mainland England to take over the vineyard. They've just finished building a shepherd's hut that became available to rent in June.
Plus ferries also run regularly between the islands, especially to St. Mary's, the main island, which is only a 25-minute journey away -- so daytrips can be made to St. Martin's too.
Most visitors spend their time island-hopping but barely get to know St. Mary's. It may have the most inhabitants -- although still below 2,000 people -- yet stray away from the pint-sized capital, Hugh Town, and you'll barely meet anyone.
And it's every bit as beautiful as the other islands. Chy Carn is a spacious, light-filled house, perfect for a big family to rent. There's also the Star Castle Hotel, a 16th-century star-shaped castle.
A 10-mile coastal walk circles St. Mary's. Part of this route is around the Garrison which bulges like a balloon from the rest of the island, separated by an isthmus.
Here, walls built during the 17th-century English Civil War edge the ice-blue apron of sea, once shielding these far-flung islands from French and Spanish aggression.
The islands have something to offer in all seasons (except perhaps winter when almost everything closes).
In the fall the land is covered with ferns and heather. In spring these are swapped for narcissi, gold gorse and yellow fists of aeonium flowers which sprout up from the plant's purple rosettes. In the summer, islanders say there's so much color it's hard to know where to look.
St. Mary's coastal walk passes the Innisidgen Bronze Age burial chambers -- two tombs made from huge silver slabs of granite and covered with mossy mounds. Known locally as entrance graves, there are over 80 of these on the Isles of Scilly. They're unique to these islands and West Cornwall on the mainland.
Nearby is a rope swing on which children (or adults) can soar over the ocean.
The Scillies have a climate that's typically more temperate than the rest of the UK.
The Isles of Scilly have always needed protection -- not just from enemies but also from the tempestuous seas which surround them. Further around the headlands is Peninnis lighthouse, its hooped metal base like the underskirts worn under Victorian dresses.
The ocean's grim legacy can be found in the atmospheric graveyard of Old Town's small church. Here, among an Eden of palm trees and wildflowers, many headstone inscriptions reveal the dead who were lost in the numerous shipwrecks which still ring these isles.
These islands have had a long and fascinating past, with centuries of antique leftovers. Their history is particularly evident on Teän, another abandoned island near St. Martin's.
The islands' sole archaeologist Katharine Sawyer, who runs Scilly Walks, takes groups out here, or to other islands like St. Helen's (which has a medieval hermitage) or Samson.
The Isles of Scilly have the biggest concentration of ancient monuments anywhere in Britain, she says.
"It may not be that more were built here but that their preservation has been so good," Sawyer explains. "We've always had a small, remote population so less pressure on the land, and never had commercial quarrying."
On top of Teän's Great Hill -- where, in springtime, gorse releases the scent of coconut -- is another Bronze Age burial chamber; plus views of vanilla-white bays and scribbles of turquoise swirling into the sea's deeper blues.
Back down again you can stroll inside an 18th-century cottage, the occupants of which remain unknown.
Thin footpaths weave past remnants of a fifth-century hut, ancient field boundaries and an eighth-century medieval chapel, still with its holy water stoup. Miniscule dwarf pansies hide among the dune grassland; these native rarities are found nowhere else in the UK.
With sustainability of fishing -- and the damage that big fishing trawlers are doing to fish stocks and the ocean floor -- currently a hot topic, it's heartening to know that these islands are surrounded by seas protected by strict laws.
Amanda Pender is joint-owner of Island Fish -- a family business on Bryher, one of the smallest of the inhabited islands.
They've been fishing off this coast for generations, supplying freshly caught shellfish throughout the Isles of Scilly and have a little, wood-slatted restaurant.
"My father has been keeping daily records since 1972," Pender says. "And the fishing stocks are just as good then as now."
The Abbey Gardens is a colorful showcase of Mediterranean vegetation.
Another island is Tresco, home to the Abbey Gardens. Here are over 3,500 different plant species which grow from all five of the world's Mediterranean climate regions: puyas from South America and South African proteas to name just a few. Golden pheasants -- a flash of rainbows with headdresses that resemble Egyptian Tutankhamen masks -- dash about in the gardens. Tresco is car-free -- you can hire bicycles from the Tresco Bike Shed. Ride to Pentle Bay, and you'll see black-and-white oyster catchers with long orange beaks. Many birds on these islands are fearless, since there are none of the mainland predators like foxes, weasels or snakes.
The song thrushes alone are far more abundant than elsewhere in Britain. Here, in this unique ecosystem, birdsong is so vibrant, it's like listening with headphones.
The islands are home to Richard Larn, a remarkable 90-year-old leading expert on shipwrecks -- recognized by Britain's Queen Elizabeth for his services to nautical archaeology and marine heritage.
Larn tells CNN Travel that the most interesting shipwreck is HMS Association, smashed off the Isles of Scilly in 1707 at Bishop Rock, where one of the most isolated lighthouses now stands exposed to the wrath of the open sea (St. Agnes Boating Wildlife Safaris offers trips here.)
Hundreds of years later Larn was involved in an expedition to find the wreck, a near-impossible task as these waters are deep. Eventually they struck gold -- literally -- as thousands of gold coins were discovered too.
Surprising treasures still wash up onto the beaches today: these islands are a mysterious place where we still don't know what lies under the waves, nor undisturbed below our feet.