The blue sapphire sea in front of me suddenly darkened. Nearly 30 metres down in the depths of the Indian Ocean, I knew we were about to come across something big. I just didn’t know what.

As my four fellow divers and I swam closer, the grey, amorphous mass slowly began to take shape and come into focus.

It was the cold glint of a steely eye that gave it away, an eye in such a strange position and on such a oddly-shaped body, it could only be one creature.

Then suddenly, they were upon us: hundreds and hundreds of hammerhead sharks. Above me. Below me. To the side of me. Silhouetted against the sky and as far as the eye could see.

I stopped breathing, not through fear, but from sheer awe and wonder. The world slowed as I tried to savour every moment, remember every detail. I may have only been diving a couple of years but I knew this was something special, something few had ever seen.

We hung in the water, motionless, letting the hammerheads swim around us. Naturally shy, although potentially ferocious creatures, they gave us a good few metres of space and carried on their journey.

Each hammerhead sharks gave us a few metres of space. Then suddenly, they were gone. Photograph: Stephen Frink/Corbis Photograph: Stephen Frink/Corbis

And then, just as suddenly, they were gone.

Back on the boat, even the dive master in charge of our expedition just off Durban, South Africa, was impressed. "Never seen that many before," he said. "At least 200."

Eight years, and more than 150 dives later, I still remember those precious few minutes like they were yesterday. It’s for moments like that that I dive.

Because, no matter how many times you dip beneath the waves, you never know what you’re going to see. Or when you’ll see it. When we came across that school of hammerheads, we’d been down for 40 minutes, seen nothing and were getting bored. Then they magically appeared.

Dive the same site 10 times, and you’ll have 10 very different experiences. One day, you’ll look in a little cave to see the huge eyes of an octopus, the next a cuttlefish will scoot past you or a moray eel will waft out and give you the evil eye.

The sheer amount of wildlife under our waters is incredible. I’ve seen hundreds of crabs crawling across the sea floor just off Weymouth, had cleaner shrimp dance on my hand in Malaysia and diced with death as a tiger shark scoped me out as a possible meal in Egypt.

I’ve choked back tears as a turtle allowed me to swim alongside her in Borneo, stroked the tail of an angel shark lying buried deep in the sand in Lanzarote and chased after a myriad stingrays all over the world.

But it’s not just the creatures that fascinate me. For our oceans – both close to home and further afield – have many other wonders to show us. Mile upon mile of pristine, candy-hued coral stretching out under the sea like a Monet. Forests of what look like white Christmas trees shooting up from the seabed. Caves like cathedrals and stalactites 30 metres high. It’s endless.

Bracing British waters

Diving in British waters has its own special charms. It's challenging, because of the cold, tides and lack of visibility. I learned the hard way, choosing a wetsuit instead of a drysuit in water of just 13C. My teeth chattered the entire time. But a dive down to HMS Hood, which was scuttled to protect the entrance to Portland Harbour in Dorset from German submarines in 1914, is an extraordinary journey.

The thrill of descending down the shot line to the wreck (unlike in warmer climes, you can rarely see them from the surface), and then seeing it loom out of the blackness is unsurpassable. At first everything looks grey and dull, then you shine a torch on the barnacles and anemones that have grown on the massive hulk over the years and they light up - pinks, greens, yellows – like an underwater rainbow.

Who wouldn’t want to weave around the Bedford trucks and Enfield motorbikes that were in the hold of the British supply ship, the HMS Thistlegorm, when she was sunk by the Nazis in 1941? Or fight through hundreds of bat fish to see the steam train that fell off her deck when she was sinking?

It’s the sheer majesty and serenity of the ocean that appeals to me. The moment you leap off the boat, you’re submerged into a different world, an utterly silent, often eerie world, with no idea what you’re going to find.

The deeper you go, the more you leave your cares behind. Your frenetic lifestyle is left on the surface, to be replaced by one where everything moves in slow motion. All that matters is you and your dive buddy: pointing things out, covering each other’s backs and communicating without saying a word.

Then afterwards, in the bar over beers, swapping stories of what you’ve seen from your ringside seat in the world’s biggest aquarium.

And also, what you haven’t. For it’s that which spurs you on. There’s so much I haven’t seen yet and I’ve no idea when I’ll come across it.

Top of my list is a manta ray. Or a whale. Or a great white shark. Actually, on second thoughts, maybe just the first two. After all, part of the thrill of diving is living to tell the tale.

Do you love scuba diving? What attracts you to the water?

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