Visitors travel to Tahiti and nearby islands in search of white beaches, azure ocean and swaying coconut palms. But in fact, there are few beaches of note on this main island, and the sand you'll find is often of the black volcanic variety with rich, dark water. Nothing wrong with that of course - but it may not add up to the traditional scenes of tropical paradise the way you envisaged them.

Indeed, beaches as such are in limited supply on many of Polynesia's islands. So, where are those perfect places with bright white sand, translucent sea and green palms? For scenes like these, atolls are where it's, er....at.

Atolls are formed when volcanic islands sink into the sea over millions of years. When these eventually disappear beneath the waves, all that remains are the remnants of the reef that surrounded them. Over more aeons of time, these remnants build up higher with various deposits and dead stuff from the sea. So, what we're eventually left with are huge, low-lying, donut-shaped pieces of land, consisting of coral sand and, usually, plenty of vegetation on top.

Just an hour north of Tahiti by plane, the Tuamotus are a group of spectacular atolls, some of them dozens of miles across, and it is on these dramatic stretches of sand that you'll find get those dreamy desert island experiences.

King among the Tuamotus, and indeed one of the largest atolls anywhere in the world, is Rangiroa. Measuring some 75 km across and 30 km from north to south, this giant ring of sand is so vast that you can't ever stand on one side and be able to see the other. The odd effect is of being on an infinite stretch of narrow land, surrounded on both sides by water; on one side the open sea, on the other the shimmering lagoon.

The pace of life in the Tuamotus is snail-like, even by Polynesian standards. Journey to Rangiroa and you'll need to get in sync with one of the most laid-back societies on the planet. The absence of haste is not that surprising really, given that you're truly in the middle of absolutely nowhere, with no land more than a few metres above sea-level and little industry beyond tourism, fishing and pearl production. There is, bizarrely enough, an island winery, but if you're the kind of person who dislikes being deprived of all-night restaurants and internet access, this may not be your sort of destination.

Hop in a boat from Rangiroa's "mainland" and you can be whisked within the hour to a spot known as the Blue Lagoon, a does-what-it-says-on-the-tin location if ever there was one. It sits in the corner of the main Rangiroa lagoon and, in keeping with its outlandish beauty, this Lagon Bleu seems to creep out of nowhere.

You'll be approaching on a rambling old speedboat over choppy seas. When the captain slows the engine and navigates into some pancake-flat shallows toward the lagoon, your eyes might not quite believe the view.

Encircled by motus (little islands of coral sand, usually covered with a picture-perfect wee forest of palm trees), the ocean here is shallow and clear, and therefore a bright shade of blue. There's a reef right around it, which keeps the waters nice and calm, effectively creating a lagoon within a lagoon. And it's very, very blue.

Whichever direction you point your camera, the Blue Lagoon's scenery will leave you dumbfounded. Surely, places this perfect only exist with assistance from Photoshop? Apparently not.

A sumptuous fish barbecue is the icing on the cake of an excursion here, but more fun and games to follow as you head homeward. Lingering in the lagoon you'll have spotted a shark or three - harmless ones just a few feet long, zipping around your knees and keeping dorsal fins on show. But when your boat edges out of the lagoon in the late afternoon, it will be followed by dozens or even hundreds of sharks - and bigger ones than you'll have met earlier in the day - tagging along in the hope of being thrown some dead fish. The sight is a spectacular one but the sharks are pretty much harmless, and braver swimmers can even hop into the water to get up close and personal with them. Keep your eyes peeled when travelling to and from the lagoon and you should get to spy dolphins, stingrays and flying fish doing their respective things.

Rangiroa is the mother of all atolls in this part of the Pacific but there are dozens of others to choose from, depending on how many flight and boat connections you're willing to put up with. Smaller Tikehau, just next door, is a popular alternative to Rangiroa.

Wonderful beaches can also to be found on motus, which sit in the sea just off many of Polynesia's islands and have the same ecological make-up as atolls: coral sand + palm trees + reef = postcard perfection. Among the more spectacular are those off Bora Bora, the honeymooner-friendly island whose fairytale mountain peaks, clear lagoon and extravagant over-water hotels have to be seen to be believed. From here and other islands, you'll be able to take a day trip out to a motu for a picnic - often eaten at tables actually within the shallow water, so that you can cool your feet as you feast.

(Visit www.tahiti-tourisme.co.uk for more info on Tahiti and her islands. Click here to search for Tahiti flights with Air Tahiti Nui.)