Scuba diving at night is an odd activity. Rose Bay commuters confirmed this with their quizzical looks while disembarking the ferry from Circular Quay. They landed amidst the dockside debris of a dozen deranged individuals apparently preparing to spend their Friday night at the bottom of Sydney Harbour.

The scene obviously took the commuters by surprise - it was not every night that they headed home through a battlefield of air tanks, flippers, regulators, buoyancy vests and, well, odd people hopping around while straining to push their limbs through tiny apertures in black rubber suits.

You could see them thinking: "The water must be freezing," - "Don't they know sharks eat at night?" and "They must be bloody mad!"

But off to their warm homes they went - to the hubby, the Mrs, perhaps some beer and a pizza - while we wet-suited cretins got ourselves and our gear on board Yves Moulard's charter boat for the dark ride out to North Head.

The anchor exploded in a blaze of phosphorescent light before plummeting about 15m to the boulder-strewn bottom a stone's throw from Sydney's sentinel sandstone cliffs. A thin slice of moon added a decorative touch to the night sky but failed to shed much light on our position in this shadowy recess. We even had trouble identifying the features that gave the dive sites off these cliffs their names - The Wall and Old Man's Hat.

The first group was soon ready to take the plunge and those of us remaining on board breathed a sigh of relief. Oh well, if there is anything big and hungry down there, we thought, dinner is on the way.

We were all experienced divers and had all witnessed the marine wonders that only show themselves at night. And some may have denied any apprehension about entering the weird underwater world at night, but I don't mind admitting a bit of paranoia when it comes to my worst nightmare - shark attack!

Fortunately, my common sense usually gets the better of my fear. There are people diving somewhere off Australia every single night. And how often do we hear about some poor soul being mauled by a shark? And how often is it a scuba diver? And have you ever heard about a scuba diver being eaten by a shark at night? So there you go, if you have any common sense scuba diving at night is not at all odd.

That said, the part I don't like is floating around near the surface - I've seen Jaws too many times. For starters you have to jump in the water and everyone knows that sharks are attracted to things that go "splash" - day or night!

Then you point your skinny torch beam into the vast liquid void and realize that - no matter how you wave it about - only a tiny percentage of the black abyss will ever be revealed. But common sense prevails.

Off North Head on this mid-October night, the visibility was about 10m - as far as the torch beam extended. And yet it was a jellyfish less than 1m away that first captivated my attention.

Bound in my rubber suit I was immune to its sting. While waiting for my buddy to join me I studied it's transparent, bulbous body pumping like a bellows as it swam against the tide. Its tentacles were about 30cm long and they danced in unison to the swimming motion.

In daylight the jellyfish would barely have been visible, but at night it was transformed into a creature of rare beauty, like a fine crystal lampshade illuminated. My buddy was now with me and I shared this marine wonder before descending in search of more.

On the bottom we encountered an army of echidnas on the march - dozens of sea urchins extending hundreds of bristles into the passing current, filtering microscopic marine organisms into their fleshy midst.

Our weight belts had done their job getting us down so we now injected our vests with a blast of air to achieve neutral buoyancy before flippering along a metre or two above the bottom.

Massive boulders had long ago tumbled from the cliffs above onto the sandstone ledge we explored. The sides of these boulders were adorned with multi-coloured soft corals and sponge gardens. The variety of shapes was fantastic - stranger than anything a kindergarten class could create with a tonne of plasticine.

Up ahead four divers hovered above something of obvious interest. Their torch beams merged in a large sand-filled crevice between two rocks. We joined the group and studied the crevice for a few moments before discerning a rather large Wobbegong shark, neatly camouflaged and apparently quite asleep despite all the attention.

These sharks are supposed to be harmless but a few years back a lifesaver emerged from the surf with one securely locked on his arm. The shark's head had to be severed before the lifesaver was freed. Fortunately, Wobbygongs are also known as gummy sharks so the lifesaver was not seriously injured.

Back on the boat, with a remnant drop of adrenalin bubbling through our veins, we swapped excited stories about what we'd seen. Sightings included a gigantic lobster, cuttlefish and numerous rainbow-coloured fish, adrift in a technicolour dream.

It was after 9pm when our French skipper raised the anchor and cruised across to Bradley's Head where our hunger was sated with prawns, barbecued lamb, cheese and wine. From here we had a great view of the Opera House and the Harbour Bridge.

What did you do last Friday night? Nothing special - that's odd!