With pounding heart Derek Cheng holds his ground to admire the beauty and grace of a bull shark.

Your body does odd things when a shark closes in on you. It seizes up, pleads for urgent intervention. Your "fight or flight" instinct bursts forth, and you quickly discern that flight is preferable to competing against giant jaws that could slice thtrough metal.

But you know that swimming madly for the shore would be like dangling an enormous carrot in front of a donkey. Except that the donkey has a menacing, all-consuming mouth. And it can easily reach the carrot whenever it wants.

So you force yourself to disobey your instincts. You freeze and pray that the beast will leave you be, while contemplating which limb is expendable. Ankle? Forearm?

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The warm waters off the coast of the Yucatan Peninsula, Mexico, are prime real estate for bull sharks. Our guide must have thought we were a rare kind of stupid. "Don't lunge at them," he told us. "Stay calm; still." Got it.

Direct provocation, not good.

We took a boat to a spot not far from the shore, and flopped into the ocean, sinking 20m to the sandy bottom.

We each took up a prayer-like position, our knees nestling into the sea floor. In goggles and with a regulator in your mouth, your sensory world is stifled, vision hazier, hearing muffled.

Nothing happened. We looked like an underwater cult on the brink of mass suicide. I was beginning to hope we weren't going to see a thing, when the guide tapped me on the shoulder.

As I turned, a mama bull shark came into view, gliding effortlessly. She sported a light grey overcoat and a naked, white underbelly. The size of a decent coffin, she had two baby sharks hugging her rear fins.

She moved fluidly, drifting her head from side to side as though she was paranoid about being followed. It was the equivalent of a mother taking her kids out for a weekend stroll through the park. Despite my chest shrinking with anxiety, I could appreciate her beauty and grace.

Bull sharks have a reputation for being aggressive and unpredictable. And big: up to 3.5m long and 315kg. That they've been known to attack hippos in Africa suggests a streak of ambition and opportunism. Their fondness for shallow waters makes them the prime suspect in most attacks on people.

Other sharks soon followed her, and we were quickly surrounded by half a dozen bull shark families. They scanned around us for food and seemed uninterested in these strange, wet-suited creatures with tubes dangling from their mouths.

Just when it all seemed rather tame, I spotted a shark about 20m away, heading directly for me. I remained statuesque. Five metres from me, clearly intimidated by my lack of movement, odd posture, and steady exhaling of air bubbles, she changed her angle and swam away.

The only real danger presented itself in the form of a colourfully striped lionfish, whose fin rays are full of enough venom to cause severe discomfort and, in rare cases, death. The guide quickly stopped me when I started to stretch my fingers towards the pretty fish.

The endless subterranean caves and caverns — called cenotes — in the Yucatan present a different kind of threat. The narrow chutes are sometimes barely wide enough to accommodate a beach ball.

Cenotes are formed when limestone dissolves and rock ceilings collapse. They are often perfectly rounded pools that connect to vast underground caves, sometimes up to 140km long.

Negotiating them as the vortex continually narrows feels like a slow march to underwater oblivion. Not the best place for a panic attack, as my friend Nina discovered. Flight mode seized her and she had to make a frantic swim for the closest air pocket to remove her mask and regain her composure.

Cenotes have little fish life, but infinitely clear water and hanging gardens of stalactites, providing a magnificent seascape. It is a different beauty to a sharkfest, but no less majestic or impressive.

CHECKLIST
Getting there
Air New Zealand and partner airlines fly from Auckland to Cancun, via LA, San Francisco and Houston. Economy Class fares start from $1183.

Details
There are numerous dive companies on the beach. A dive usually costs
$50 to $60.

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