Warning: If you tell someone you're writing a book about professional mermaids their first impulse is often laughter. Don't expect to be taken seriously. Not yet.

"What is a professional mermaid?" That's the first question.

In 2005 Kazzie Mahina, a 27-year-old professionally trained dancer, walked into Sydney airport with a large brown bag slung over her shoulder. She'd sold everything she owned and bought a one-way ticket around the world.

"It was a very Saturn return thing to do," she says.

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Her handmade shoulder bag was cut to shape, two giant butterfly wings protruded from the top. "I used to get looks, people just could not figure out what was in that bag."

"Is it a harp?" they asked.

In airports across the world, little girls clocked this babe with hair down to her bum passing by, bag over her shoulder, then turned to their mothers and said, "Mummy, there's a mermaid." The mothers corrected their daughters, "don't be silly," then Mahina pulled out her business card. On it: a photograph of herself as a mermaid.

And in the bag? Her tail. She unzipped it and let the girls touch the silicon scales. Her tail felt fishy, raw and rich with a coral colour. Mahina made it in the workshop of a special effects designer on the Gold Coast, using teaspoons to press each gooey scale into shape before the silicon set, then attached the dried scales to a neoprene base, and decorated the tail with gold-flecked paints. The process took months. "It really looked like I had just stepped out of the ocean, peeled off my tail, shoved it in the bag, magically waltzed to the airport, and bought a ticket to find more turquoise waters, and why not?"

Because mermaids don't exist? Wrong. Mahina is one of the forerunners of the current mermaid phenomenon. In the early to mid 2000s the first professional mermaids surfaced on the internet. It's a self-made career, carved out by pioneers like Mahina and her friend Hannah Fraser, an underwater model and ocean activist, and Linden Wolbert, a PADI master scuba diver who runs her own educational ocean web series for children: Mermaid Minute. All three women are beautiful blonde trailblazers who've helped establish the mermaid as an advocate for marine conservation and created a new industry including a market for swimmable fabric and silicon mermaid tails. Mermaids have their own lingo too — check your mersona and invest in a good merwrangler for getting in and out of that pool and don't get fin-slapped honey. (P.S. Your shellphone is ringing.)

Sexism, silliness and the special-effects industry shape the world of the professional mermaid and it's much deeper than you might think. When I asked my Twitter followers if anyone knew of any professional mermaids in New Zealand, I got sent a picture of Wellington's Mermaid Strip Club. Ha, ha. Look through the portholes of The Wreck Bar in Florida and you could watch MeduSirena and her burlesque Aquaticats glide by as you sip a martini after hours, but most professional mermaids are PG only. In the last American Census, 1000 people listed mermaid as their profession. And the movement is global.

Professionals work "wet" and "dry" events; train as free divers for underwater photoshoots at depths of 40+ feet, flaunt their tails in Pride Festivals; blow bubble kisses in portable glass tanks at renaissance fairs or sit in clamshells for extensive meet and greets; they dolphin kick alongside whale sharks, dive through aquariums, then surface on Instagram; the professional is often an entrepreneur and an edutainer, who donates her time to community charities, runs mermaid swimming schools, and grants the wishes of children, popping up in the family pool at a birthday party, "surprise!" And they do it all with their feet pushed into a monofin.

Blip-blip-bloop. For a year I've been interviewing professional mermaids on Skype. Every time my computer bloops I know a mermaid is online. Or a merman. The plural is merfolk or "mer" for short and like whales and dolphins, they swim in pods. The Mernetwork is an online forum for professional and recreational mermaids where members can meet others in their affiliated pod and organise group meets at the local beach or pool. The bottom of a mermaid's tail is also called a fluke. But is it a fluke that mermaiding has taken off as a career in our climate-change challenged, post-truth world?

"The mermaid belongs to every woman on this planet," Mahina says. She's driven up her gravel drive, in Byron Bay, to get enough signal for our Skype. Mahina's been in RedFoo's Let's Get Ridiculous music video and illustrated by the Wall Street Journal, but is a "quiet achiever" who will turn down publicity if it just going to make her look like a "freakshow". She lives away from the city, saying, "I can't be without nature".

Mermaiding is a 15-year journey that started in 2004 when she did a local commercial alongside Hannah Fraser. They both dressed as mermaids and swam with dolphins. Fraser, now based in LA, is arguably the most recognised professional mermaid in the world (other mers call her The Queen). But for Mahina, mermaiding was always an elemental connection she wanted to share.

"The Mahina MerFin has been my vehicle to share the joy and connection with the ocean and to entice people back into nature," she says.

In 2006 she began prototyping a tail before there was any commercial market for mermaids. As a surfer, she wanted to design a bespoke monofin to promote mermaiding as a water sport.

"I went into it from a surfboard shaping point of view. I wasn't just cutting out a pretty shape and then giving it to a factory saying, "can you make me something like this?"

I was at the factory designing the right millimetres for the foot bed, and the percentage at which the fluke tapers off to get the right propulsion and movement through the water. There was also a lot put into the safety release of the feet."

The end result was the Mahina MerFin, an award-winning product made from recycled rubber that has retailed in 65 countries worldwide, including Harrods, Urban Outfitters, Nordstrom and Smith & Caughey's.

For the Auckland product launch Mahina made an underwater appearance in her tail at Kelly Tarlton's aquarium in the shark tank. She was four months pregnant and waving to children she actually couldn't see. "The hard thing about aquariums is the visibility is not great."

Mermaids don't wear goggles; it is an athletic profession. The Mahina Merfin is now a coveted women's surf-and-swim accessory, yet getting the product taken seriously in the male-dominated world of water sports is tough. "The minute they hear 'mermaid' they just associate it with sparkly fantasy tail hoo-hah."

Yet, Mahina has stuck to her principles, refusing to retail her MerFin through SeaWorld, calling it, "blood money". Did you know when SeaWorld first opened in 1964 it featured a mermaid show?

Today's professional mers are conservationists. Mahina has partnered with Tangaroa Blue Ocean Care Society specialising in local beach clean ups. A role model for young girls and teens, Mahina also had her own eco column in the girl magazine Total Girl.

"I'm more tuned into the essence of what the mermaid symbolises rather than being a real-life mermaid. My Instagram is followed by an organically growing movement of women swimmers, free divers and surfers. There are a lot of women picking up what I'm putting down."

— of the Vancouver pod — is one of them. Her family enrolled her in swimming lessons at 2 years old and got her first mermaid tail at 16. A lifeguard and an audio-visual tech, she's also a recreational mer, who started out in an "Ariel" green tail. She swivels the camera round and reveals 19 fabric tails lined up on her wall. Nerdmaid has her own YouTube channel reviewing mermaid tails and monofins — the Mahina MerFin is her favourite. "It's got such massive power to it.

"Basically, when I first heard about mermaiding I was, like, 'This sounds freaking rad'. Because I love to swim. So I was like Mermaid tails? That's something I could get behind."

Nerdmaid has swum in a mermaid tail in a glacial lake in Alaska, but her favourite place to swim is her local pool five minutes from her house.

"There are times when I just need to swim; it's a meditative thing. I really enjoy just being in the water and having that feeling of weightlessness."

"Real mermaids don't wear tops and real mermaids don't have knees," are two of the most common heckles the professionals get. "We deal with a lot of sexual harassment, men try to grope us in front of children and you can't walk away because you're in a 30-pound costume. It drives me nuts," says Stephanie Brown aka Raina Mermaid. Raina is another industry pioneer, who first became interested in becoming a mermaid after seeing the blockbuster movie Splash.

"I saw Splash again as an adult during a difficult time in my life. I realised the tail was a costume, not CGI and so I went down this Google rabbit hole …"

Raina says the mermaid boom is three-tiered. "You have the resurgence of feminism, the rise of social media and a tail-making industry that has become so much more affordable."

Mermaiding taps into a primal part of humanity. "Put on a pair of wings and you're not going to fly, but if you put on a mermaid tail you're going to swim — and you're going to be fast. Even weak and nervous swimmers find it empowering."

Raina has her own YouTube channel with more than 9600 followers and 400+ vlogs on her life and work in a tail. She runs her own business Halifax Mermaids with 12 mermaids on her payroll in Canada, where it often snows. The challenges are significant: a smaller population, a colder climate, fewer swimming pools. Twenty-five per cent of her business comes from tourism; Raina also does various talks and events for adults, but approximately half her clientele are children.

"I have created a curriculum about the environment that we use for different age groups. We do a whole programme around plastics; we talk about the different ways to reduce the amount of plastic we use and we explain what micro-plastics are, because kids understand 'don't litter', but they don't understand how something gets into the ocean and then breaks down and gets into the food chain."

And mermen? "We live in a world that is very judgmental of men doing anything seen as feminine, or men working with children," says Raina.

In 2016, she conducted a survey of mermen in the community to bust some stereotypes. "Of the mermen she spoke to 40 per cent were straight and only 9 per cent used their tails professionally."

"We all have nightmare stories about people who just want to make a joke out of what we do," says Jack Laflin, or Merman Jax, who runs a boutique LA-based company Dark Tide Productions, creating special events and is one of the most successful professional merman in the industry. His company of stuntwomen, freedivers and athletes create performances and events that include mer work.

Mermen are often requested by the LGBT community and the mermaid is also a powerful symbol for the Trans community.

"Mermen are a harder sell, but in the right niche category I work more than other mermaids."

A competitive swimmer, descended from Filipino abalone divers and Irish whalers, Laflin first wanted to be an actor, but because of his perceived ethnicity was typecast as either "sexy club guy" or "gangster criminal". Now he's a professional merman inspired by the African Goddess Yemaya.

"Lots of black mermaids identify with Yemaya because they don't see representations of themselves on TV except as extras, usually the main mermaid is a beautiful blonde."

Some people will tell you that mermaids don't exist and that climate change isn't real. Don't believe them. Because I've heard the mermaids skyping and they're off the hook.

Megan Dunn is currently working on a book about the rise of professional mermaids.