He's not a big shark - just an average grey reef shark. Ordinarily, you wouldn't give him a second glance other than to admire the streamlined perfection of his form. He's been circling for a while, steadily moving closer, but still this is not unusual behaviour. We watch with growing concern as he begins to swim in short, erratic bursts, then suddenly turns straight towards us, arches his back, drops his pectoral fins and rushes us. It's classic posturing and it means: 'I don't like you being here - get out of my territory. Now!' It's over in a blink an eye and he veers off a few metres in front and circles back to his original position. If we were to ignore his warning he would do the same thing perhaps once or twice more, then finally lose his temper and bite. But we know who's boss in the marine world, and withdraw from his patch of reef. We are diving in a remote atoll in the northern Marshall Islands, one that until two years ago had never seen scuba divers and has seen precious few since. So let's consider the shark's point of view. He has probably never encountered a scuba diver before, so to him, we're an unknown threat. It's no wonder he's upset. Most diving enthusiasts who travel regularly to Asia's long list of dive destinations harbour a dream to discover a hidden paradise: to be a true dive pioneer. With the blooming of diving's popularity in the past 15 years, however, this will remain a dream for the majority. Dive operators have explored and set up dive resorts or live-aboard trips in nearly all accessible locations in the Asia Pacific region. But not all. The name Rongelap atoll will probably mean nothing to you - but how about Bikini Atoll? Also part of the Marshall Islands, this was the site of the infamous Bravo atom bomb test by the United States in 1954. The fallout from that - and subsequent - explosions drifted southeast 100 kilometres to the Rongelap atoll group, and its small population of islanders was evacuated. They were erroneously allowed to return long before radiation levels were safe and were taken off again in 1985. It was only in the summer of 2002 that the region was finally declared safe (radiation readings are now no higher than at the average city airport). Since then the Rongelap Atoll Local Government (RalGov), based in Majuro, has realised it has a rare opportunity to use what is essentially an untouched marine environment. Paradoxically, the enforced isolation of these atolls has resulted in the preservation of immaculate atoll reef ecosystems. (Ailinginae atoll, 32 kilometres southwest of Rongelap, has been nominated for UNESCO World Heritage listing.) A small, exclusive resort is being built on environmentally friendly principles and is scheduled to open this year. In the meantime there are luxury live-aboard dive trips around the atoll - and to Ailinginae when the weather allows. The MV Oleanda, a 40-metre luxury live-aboard boat, offers multi-day dive and eco-cruises, but the most tantalising is the week-long Exploratory Dive Expedition. In a shallow sweep of a bay the Oleanda halts at one of five fixed moorings in the lagoon. The dive is a simple one, 'just to get your lungs wet', as expedition director Wayne Fox says. We circle the dive site, a coral bommie rising to five metres below the surface, covered with a range of healthy hard corals. Dense thickets of staghorn coral surround the bommie's base, forming a protective home for juvenile reef fish. The diversity of marine life and lack of damage illustrate the area's pristine nature, and as if to punctuate this point we find giant clams larger than any I have ever seen, a metre long with shells gaping and vivid mantles bulging. The presence of these super-molluscs, which have been wiped out in so many Pacific islands, bodes well for this ecosystem - and our coming week of diving. Although the Oleanda crew has explored Rongelap extensively to find the best dive sites, its sheer size dictates that innumerable tracts of reef and lagoon areas remain undived. Herein lies this trip's great attraction: genuine exploratory diving. Even a long-time dive enthusiast with hundreds of logged hours underwater can't help but be excited. What might be down there? Sleeping sharks? A coral garden? An old shipwreck? Way out in the lagoon we visit a lone coral head; there are a few of these marked on the charts but none has ever been dived and Wayne is interested to see what surprises they may be hiding. We will explore the bommie, note any unusual physical or organic characteristics of the site and decide on its rating for future dive groups. We will also have the honour of deciding a name for the location - if we can agree on one. I sight a small group of chevron barracuda as I descend, then a few grey reef sharks, an eagle ray at a distance, a frightened, almost pure white porcupine fish and many enormous giant clams. The Oleanda settles into its itinerary, sailing in a zig-zag fashion anticlockwise around the lagoon. We make lagoon dives, ocean-side wall dives on the outer reef and dive through the channels, or passes, that flush water in and out of the lagoon with the tides. As is generally the case in tropical waters, if you want to see major fish action and a full bloom of vibrant coral growth, then the outer reef, in particular the passes, are where you want to be. There the reef is at its prodigious best, and consequently the fish action is most dramatic. Small Boat Passage is an impressive pass, narrow and shallow, resulting in ripping currents. A sandy bowl at the mouth has been named The Aquarium, for obvious reasons. The plethora of fish species includes a nursery of baby reef sharks, though there are plenty of well-fed mothers too. Letting the current take us, we fly past steeply sloping walls festooned with hard corals, anemones and sea cucumbers. Swimming with us are blacktip, whitetip and grey reef sharks, eagle rays, napoleon fish, green turtles, schools of blue trevally and midnight snapper. It's a real treat, and everyone is all smiles and stories on leaving the water. The diving has lived up to expectations, but what we hadn't expected is the level of luxury and quality of service on board Oleanda. You expect to be well looked after on this type of trip, but throughout the whole week we have been overwhelmed by the smiles and impeccable service of the crew. Food of a quality usually reserved for five-star restaurants has left us with kilos to lose when we return home. At least my reserves are full for the journey. Way to go: The journey to Rongelap takes two days by any route. Continental Airlines ( www.continental.com ) flies to Majuro from Guam on Mondays, Wednesdays and Fridays, returning on Tuesdays, Thursdays and Saturdays. There are stops at Chuuk, Pohnpei, Kosrae and Kwajalein Atoll before Majuro. It's a long flight and perhaps a better (and no less expensive) alternative is to fly to Hawaii, stay overnight in Honolulu, then fly down to Majuro with Continental or Aloha Airlines ( www.alohaairlines.com ). Either way you must stay a night in Majuro before flying to Rongelap with Air Marshall Islands (AMI). How to go: The seven-night/eight-day Exploratory Dive Package costs US$1,100 to $1,700 depending on your cabin on the Oleanda. Included are airport transfers, a night's accommodation at the Marshall Islands Resort in Majuro and everything needed aboard the boat (except alcoholic drinks). Rongelap Expeditions ( www.rongelapexpeditions.com ) will book your AMI ticket for you. When to go: The best diving season is June to October. Northeasterly trade winds early in the year bring swells and blustery conditions that can limit site access.