They appear out of the cold, murky waters - huge, dark shapes, their noses flat and broad, their eyes flinty in the filtered light. They glide past, silent in the deep ocean, sleek and menacing, before disappearing once again into the gloom.
Although the danger is minimal, you cannot help but breathe a sigh of relief. This is our first brush with a hammerhead shark in the Galapagos Islands. It is not something you readily forget. There is plenty more down here to see, from sea lions that frolic in divers' bubbles to penguins and huge, majestic turtles, but nothing compares to that first brush with the Galapagos' most famous underwater resident: the hammerhead shark.
We are scuba-diving near Isla Tortuga, one of the smallest and oldest islands in the Galapagos, a sunken volcano that has been reduced over time to a horseshoe shape. The ocean around here is cold and murky, often affected by strong currents and surface chop.
"Diving in the Galapagos is many things," says our instructor, Paco, "but it isn't easy."
Yet people flock to this island group in the middle of the Pacific, because of its uniqueness. Where else can you dive with sharks and penguins? Where else would you see turtles in such abundance? Where else would you find sea lions as eager and playful?
The famed Galapagos Islands, cradle of the concept of evolution, are well known for their abundance of land and air-based life. People know the tortoises, the likes of Lonesome George, that wander through these volcanic outcrops. They know the mohawked marine iguanas. They know the blue-footed boobie birds, the red-throated frigates and the hawks.
But the secret of the Galapagos is that there is just as much under the water as there is above it.
Our boat is rocking in the heavy seas next to Isla Tortuga as we don thick wetsuits and prepare for our dive. "We won't see many colours down there today," Paco warns, edging to the side of the boat, "but, to be honest, that doesn't matter. We're here for the big stuff."
The big stuff: that is the turtles, the sea lions, the huge schools of mackerel, the barracuda, the manta rays and, of course, the hammerheads.
There is barely any coral at this site, and no small wonders like sea anemones or tiny colourful fish. We are here to see things that look as if they could eat you for dinner.
The best part is this stuff is certainly not limited to those with a scuba-diving licence.
The trip I am doing in the Galapagos is on the MV Grand Odyssey, a luxurious expedition vessel that places the emphasis as much on what is below the water as what is above it.
Some of the passengers are divers, but most will just be snorkelling, taking in the wonders while floating high above.
Some days you do not even need to get wet to appreciate these seaborne attractions. One of the first shore landings on the Grand Odyssey cruise is at Las Tintoreras, an islet moulded by the sea from petrified lava. While baby marine iguanas laze on the hot rocks, it is what lies in between those stone paths that is most impressive: a natural channel of water in which reef sharks, known here as "tintoreras", come to rest and breed.
A short stroll takes you past 10, 20 or 30 of the sharks lazing in the warm waters only an arm's length away. It is our first taste of the incredible interaction that humans can have with the animals of the Galapagos, animals so unused to the threat of predators that humans are seen as little more than a curiosity.
This phenomenon will be on show once again on day two when we finally get into the ocean.
We are anchored at a place called Punta Morena. Up above, volcanoes such as Sierra Azul and Sierra Negra rise from the horizon, the calling cards of these amazing islands, but below is where our attention lies.
The underwater world at Punta Morena is moulded by the forces of nature, a seascape of jagged volcanic rock that is now covered in seaweed. Marine iguanas, those fantastically strange products of evolution, usually feed underwater here, but today the stars of the show are the turtles, huge reptiles that appear in front of you like ships coming out of the mist. They glide slowly past, seemingly as interested in our group of snorkellers as we are in them.
Sea lions, too, come in to check out the commotion. A colony of penguins flits through the water next to us. This is everything that makes the Galapagos great.
Actually, hold that. The other thing that makes the Galapagos great is the cup of hot chocolate waiting for us on the MV Grand Odyssey. Snorkelling in these islands is amazing, but it is not warm.
That will not, however, deter us from getting back in. By day four, our boat has moved on to Punta Espinoza, a bay carved out of a submerged lava field. There is no coral here, just an abundance of marine life, from fish to eels, penguins and iguanas. They swim around rocks that look like melted wax.
At Playa Espumilla, flocks of boobie birds fish around us while we snorkel.
I am paddling away, minding my own business, when the water explodes around me in a huge rush of bubbles as a boobie bird plunges into the ocean from high above, disappearing a few seconds later with its fishy lunch.
Then we are on to the tour's last snorkel and its highlight, Punta Egas. The waters are warm here. There are coral, colourful tropical fish, starfish, huge sea lions, turtles and Galapagos sharks.
These Galapagos sharks are smaller than their more famous hammerhead cousins at Isla Tortuga, but smaller sharks are not always a bad thing.
The writer travelled courtesy of LAN Airlines and Chimu Adventures.
GETTING THERE Fly to Santiago then to Guayaquil (7hr 30min including transit time in Quito) and then to Galapagos (2hr). See lan.com.
TOURING THERE Chimu Adventures has five-day, four-night Galapagos cruises aboard the MV Grand Odyssey. The cruise incudes all meals, twice-daily wildlife tours, snorkelling equipment, and the services of a qualified guide. See chimadventures.com.
MORE INFORMATION galapagosislands.com.
- FFX Aus