Imagine a dark, ominous shadow the size of a school bus moving towards you through the deep water. Now imagine you and nine other people are being urged by the dive master to jump off the back of the boat immediately, like wetsuit-clad lambs to the slaughter. "GUY, GUY, GUY, GUY," she yells. This is Australian for "GO, GO, GO, GO!"

Day 1:

This place feels like the end of the world...what better place to find the biggest fish in the world? Exmouth is on a long cape sticking out into the Indian Ocean, with big blue skies and red, rolling hills covered with scrubby vegetation.

After my team arrived at our hotel, part of a former naval base near the tip of the cape, we had a tour around the bend to the ocean side. There we saw Ningaloo Reef, marked by a waving line of breakers and the brilliant turquoise of crystal clear water over coral sand. Along the way we saw a family of emus, a kangaroo, a rock wallaby, and countless birds.

The hills that form the spine of cape, called the Cape Range, was formed by coral reefs more than 10 million years ago when the seas were higher. We can see their column-shaped coral structure, pocked with caves, as we drive by.

Brad Norman, our principal investigator, suggests there's fossils in them thar hills, and we all make a mental note to keep an eye out for them. As we drive by these craggy hills, I wonder if whale sharks frequented the reefs here 10 million years ago, as they do today.

Back at the hotel, we learn a bit more about whale sharks in preparation for our first swim tomorrow. Brad tells us that Ningaloo is the best place in the world to find these rare creatures, because they visit every March and April to feast on the millions of eggs from coral polyps. This explosive spawning event is a reliable source of nutrition in tropical waters where concentrated nutrients are often hard to find.

Brad's project aims to learn more about the mysterious whale sharks, how many there are in the world, where they wander, and what their life history is. We are pumped to be part of this exciting adventure. I can't wait to jump in the water tomorrow! 

Day 2:

This morning we are picked up at our hotel by a dive shop bus, along with 17 other patrons, to shuttle around to the reef side of the cape and get on the water.

Boy do we get out on the water. The conditions are unusually choppy today, and our launch bobs around on the swells like an amusement park ride from hell. And, well, let's just say I turn various shades of green and will not be able to stomach Muesli cereal again for a good while. While some of the tour patrons do a scuba dive on the outside of the reef, we jumped in the water to try out our underwater cameras and test our other equipment.

After 20 years since the last time I snorkelled in the ocean, its amazing how comfortable I still was, even in these choppy conditions. Taking photos is problematic, however, and I get several fascinating images of waves crashing over me, bubbly turmoil, and random parts of fellow divers.

Sadly, we don't see any whale sharks today, despite the efforts of five dive boats and two spotter planes. Two whale sharks spotted by the planes dive to depths before we can get in the water near them. After several false starts, pensive waiting, and several more lost breakfasts, we have no choice but to turn around.

But spirits are still high, and the dive boat operators lay out a tremendous feast as a consolation to enjoy on the way back. Miraculously, when mere minutes before I thought I would never eat again, I am ravenous.

The highlight of the day for me was stopping at a small patch reef, called a "bommie," for a snorkel on the way back to the landing. The reef here is in nearly pristine condition, owing to the isolated location and protection of Ningaloo Reef as a national marine park.

I am giddy with the numbers and bright colours of reef fish. They hover over the bommie like fireworks. I commune with several surgeonfish, butterfly fish, wrasses, parrotfish, and a pugnacious damselfish who insists on my taking his portrait.

Over a delicious dinner of burritos, we hope for calmer weather and better shark luck tomorrow. Although today is the only chance for many of the regular dive boat patrons, we will have at least a couple more days on the boat to meet the biggest fish in the world. We hear that there have also been sightings of sea turtles and tiger sharks in the lagoon near the boat landing, and we will be sure to keep an eye out for those.

Day 3:

Okay, imagine a dark, ominous shadow the size of a school bus moving towards you through the deep water. Now imagine you and nine other people are being urged by the dive master to jump off the back of the boat immediately, like wetsuit-clad lambs to the slaughter. "GUY, GUY, GUY, GUY," she yells. This is Australian for "GO, GO, GO, GO!"

Would you jump? If you had travelled half way around the world to swim next to a whale shark, and your adrenalin was through the roof, I think you would.

I certainly do, splashing into the water with legs and arms flailing, other swimmers splashing in all around me like lemmings. Then I put my mask under water and there is only the silence of 200 metres of ocean below me, the swivelling sunlight through crashing swells, and the most awesome fish I've ever seen. He stares at me with tiny, wide-set eyes and the broadest smile in nature, then turns on a dime to swim past me. His star-spangled skin is dappled with sunlight, and he moves along in slow, easy undulations. Our group hustles to line up on either side of the beast, swimming along with him in formation.

We swim with two whale sharks today, each of them several times, and that takes most of the day. The first one is on the small side, about three-and-a-half metres, and has a tendency to dive into the depths after brief visits to the surface. The second one is bigger, perhaps five metres, and fast enough that some swimmers have a hard time keeping up. We take pictures of each of them for Brad's library of whale shark individuals, recognized by their distinct spot pattern. We also experiment with measuring them, using a metre stick as a scale to determine their total length. But mostly we revel in their company. When we stop swimming after each turn, we come to the surface smiling and whooping and cheering, like we had all won the lottery or something.

Today is a distinct contrast to yesterday, and a reminder of how lucky we are to be here all week, with even more opportunities ahead for swimming with the gentle giants. Today was also different because I tried some other sea-sickness pills, which worked. Yay!

Day 4:

After two days of bobbing on the water, jumping in the water, jumping out of the water, and paddling through the water as fast as my adrenalin-pumping little legs can propel me, it feels good to spend a day on dry land. I've also gotten a lot of sun in the last two days, a shock to my pasty New England skin, so working in the lab on data entering and processing is just my speed. I even get to sleep in a little bit too, and have a delicious omelette for breakfast.

We start with uploading images into the Ecocean's whale shark photo-ID library. Each whale shark has a distinctive pattern of white spots on its back, and Brad and his colleagues have developed a computer program that uses these patterns distinguish individuals. They adapted the program from one astronomers use to identify star clusters by their pattern, a stroke of genius I place right up their with the invention of the printing press.

I work with my team-mate Angela, a debt collector from New Zealand, to select images from film clips of whale shark encounters for the photo-ID library. We need to select images that capture the area just above each shark's pectoral fin, behind the gill slits, as that is the area used by Ecocean to "fingerprint" each individual. We add to the nearly 2000 encounters recorded in the internet-based library, coming from 27 countries around the world. Watching each film clip, Angela and I relive the excitement of our first encounters yesterday. Was it really just yesterday?

After lunch I help James enter data from a survey he's been conducting on the experience of whale shark tour participants. The survey asks questions about the costs incurred by visitors to Ningaloo Reef, and their priorities and interests in coming here. James, one of Brad's talented staff, hopes that by exploring these questions it can be argued that the economic benefit of whale shark tourism far outweighs the one-time benefit of hunting, which still happens in parts of the world.

In the evening, Angela, Kathy, and I take a walk into the bush to see if we can spot some kangaroos. They have a reputation here of coming out onto the road at night to make drivers nervous. We don't manage to find any, but I see a shooting star in the north sky and the Southern Cross to the south. We also see a stick insect that looks for all the world like a wisp of tattered palm leaf. The moon is nearly full and the quiet desert air feels like velvet.

Day 5:

This morning it is calm and clear, with fluffy little clouds and a light wind coming from the southeast. In short, excellent whale shark-watching weather. By now we are old hands at the routine, unloading gear from the bus into the dingy, stowing gear on the boat, chatting up the other participants about our research, slathering on sunblock and swallowing sea-sickness pills.

Our first stop is just outside the fringing reef, where we throw in the plankton net to investigate the food available for whale sharks. The net is shaped like a large funnel, with a PVC collection trap at the end. Angela notes the GPS readings and times at the beginning and end of two samples, while Brad hauls the net in each time. The samples are poured from the trap into individual jars and preserved with formalin for later identification. The tiny creatures in each jar hardly seem like a square meal for the largest fish in the world, but that's just one of the many mysteries that surround whale sharks.

We then stop while some of the participants do a scuba dive, and I take the opportunity to do some snorkelling on the reef. I spend most of the time following a green sea turtle that scuds along the bottom at 25 feet depth, diving down now and then to get a closer look.

We see two more sharks today, as well as playful groups of both humpback and bottlenose dolphins. The first shark is about 3 metres long, a female, and idles along so slowly that we can just hover around her admiringly. This is an unusual find, as most of the whale sharks here are immature males.

The second is closer to 5 metres long, a male, and he swims right next to the boat twice as if curious. Brad and I make several efforts to measure the sharks, using the metre stick for scale, but visibility is not great so when I get far enough from the shark to get it all in the photo, I can barely see it.

Each group of ten swimmers jumps into the water with a "spotter" from the crew, who swims along with their hand raised to mark the shark's location. Only ten swimmers are allowed to swim with the shark at one time, and each group takes turns with groups from other boats in a carefully prescribed sequence that gives everyone a fair chance to "oooh" and "aaah" into their snorkels.

Today's swims are long and gratifying, and allow plenty of time for getting good identification photos. After three days in the water, we feel like we are really getting to know whale sharks better, seeing beyond their spots, so to speak.

Day 6:

Today is to be dedicated to lab work, but we don't want a sunny morning here in paradise to go by without some quality outside time as well. So Angela, Kathy, James and I start our day by driving down the beach to Turquoise Bay for a snorkel.

This bay has a distinct rip current that runs north behind the fringing reef, so we walk down the beach and swim out to catch the rip for a leisurely drift over the "bommies," or patch reefs. I see countless fish darting in and out of blue staghorn corals, an octopus nestled in a coral cave, and two green turtles sailing over the sand.

When we get back to our suite, Brad and George have set up the kitchen as a lab for identifying the zooplankton we trapped yesterday. At first, we are overwhelmed by the myriad shimmery shapes and translucent, leggy critters that fill a drop of water. But then we start recognize patterns, finding the difference between copepods and decapods and between brittle star larvae and bivalve larvae.

My favourite is an eensy weensy, transparent barrel-shaped creature called a salp, related to sea squirts, that moves about by siphoning water in one end and out the other.

Before dinner, I take some time to upload the whale shark images I took yesterday onto the ECOCEAN whale shark photo-identification library. Brad says that his fellow principal investigator Jason Holmberg has analysed images of the sharks we saw on day 3, and that one was a new record for Ningaloo Reef. We want to see if the 3-metre female we saw two days later is the same individual. ECOCEAN's library program will contact me every time that whale shark is encountered back at Ningaloo Reef or anywhere else in the world. Each encounter is one more piece in the puzzle of where whale sharks go and how many of these rare creatures are left.

Day 7:

Angela tells me she dreamed of whale sharks last night. She dreamed we were all jumping in to swim with one, when we realized it was not a whale shark but a Megadon, a prehistoric predator ten times the size of great white sharks with teeth like meat cleavers. We take this dream as an omen that we will see our biggest shark today...either that or Angela has been reading too many books about sharks.

Our last day on the water is the calmest one yet. A one-knot breeze wafts off Cape Range and the ocean is as flat and clear as a giant bathtub. We see two sharks today, one a small female, which we suspect may be the same as the one we saw two days ago, the other an even smaller male. Brad and I measure this shark with a measuring tape, and it turns out to be 3.05 metres, one of the smallest on record for Ningaloo Reef. Brad is pretty excited, as new whales on the reef mean the population still has the potential to rebound from decades of overfishing.

We have several "drops" to swim with the sharks, and I am struck by how well I now know these immense creatures. While other participants on the boat are glowing from their first encounters, I am comfortable enough with the routine to observe the details of shark behavior: how they check me out with their tiny eyes as they swim by, the slow, sinuous swing of their monstrous tail, the virtual ecosystem of other fish attached to them or swimming in the protection of their spreading fins. I am grateful that our research has involved so many encounters over the past week, something no mere tour could afford us.

On the way back to the boat landing, we stop for lunch and for a snorkel on the bommies. Most people are knackered from their swims in the deep blue, and I am too, but I figure it may be a long time before I can snorkel on a coral reef again, let alone the most pristine fringing reef in Australia. We don't come across coral reefs very often in Vermont. As I drift over the coral outcrops, the colourful fish dart in and out of crevices as if they are saying farewell with fireworks.

Day 8:

My last day in the field is a bittersweet one. In our short time together, our team has become a family, a knot of close friends with a common goal. Parting is hard, but it also marks the end of an industrious week of data gathering and a significant contribution to the project's success.

Our morning starts with an intense session of collecting all the digital photos that Angela and I contributed to and want to take home with us. As a corporate fellow, Angela will need the photos to present her experience to her company in New Zealand. I hope to publish articles about ECOCEAN's important work here in the popular press.

Then we head out for one final swim, where a 100-year-old shipwreck, the S.S. Mildura, marks the very northern tip of North West Cape. We see turtles wallowing in the shallows here, sticking their bulbous heads up for air. But they are skittish, and the visibility is low, so we just get fleeting glimpses of them underwater. The surging surf tosses us up onto the beach exhilarated and full of sand.

All too soon, we are all hugging goodbye in the hotel parking lot, and Angela and I are whisked away to the airport. After our short flight to Perth, we spend time wandering around the city before our midnight flights abroad.

My only regret is that I didn't allow myself more time to explore the other fantastic local attractions: the hiking trails of Cape Range National Park, the dolphins of Monkey Mia, or the scenic beaches of Rottnest Island. I leave Western Australia with confidence that I will be back here again, to swim with the biggest fish in the sea.

Philip Johansson works in Marketing and Communications for Earthwatch US.  He attended Whale Sharks of Ningaloo Reef team 1 in April 2006.

WhalesharkBeachEmusThe boatCoral reefReef fishSnorkellers readyMeasuring whaleshark Team accommodationPhoto ID workCooking dinnerMeasuring planktonTurtle on reefWhaleshark headWading in waterReef fishMicroscope work in labSnorkelling underwaterCoral reef fishMatching images in labView of PerthGoing homeSome of the team