They live in deep waters where light begins to vanish.
As the last remnants of the sun fade into infinity, a whale and her calf rise from the depths. They are the length of submerged airplanes, generating a churning backwash as they move.
Click HERE for a slide show of real-life mermaids.
And there, swimming among them, is what appears to be a legendary figure. In fact, she's very real.
"I'm a mermaid," she says.
That's right: a mermaid.
Her name is Hannah Fraser. She can free dive 50 feet below the sea and return to the surface on a single breath of air. But what's most extraordinary is Hannah's ability to co-exist with animals that can very quickly become unpredictable, even deadly.
"I have found a way to integrate with their environment and swim with them, and have a level of comfortability that most people will never experience in their life," said Fraser.
It is a feat of great physical daring and athleticism. Unlike most divers, Fraser wears no oxygen tank, no face mask, no warm wetsuit -- only a special mermaid tail.
"The tail gives me so much more propulsion and streamline ability," said Fraser. "I can swim much faster with the tail on than other people can without it. So really fast, strong swimmers can't keep up with me when I'm wearing it. It's like it imbues me with some kind of superhuman powers of confidence and longevity to be in the ocean."
A Young Fascination Hatches
Fraser's fascination began when, at only 3 years old, her stick-figure sketches evolved into painting mermaids. It was only a matter of time before she told her parents that she wanted to become one -- for real.
"I was a complete water baby -- throw me in a pool and I was just on the bottom playing and making up stories about mermaids," Fraser said. "I made my first mermaid's tail when I was 9 years old, and it was definitely not particularly functional. And it's quite amazing that my mother even let me tie my legs together and throw me into the pool with, you know, pillow stuffing down there. It's pretty dangerous."
Fraser has been swimming ever since, with more refined tails. In fact, she's turned her passion into a full-time profession, performing in documentaries, television and aquariums.
It may all seem effortless, but make no mistake -- Fraser's underwater adventures are not for the faint of heart.
"Swimming with the whales was one of the most unbelievable, awe-inspiring experiences that I could ever imagine," Fraser said. "I had this moment where I'm swimming out in the middle of a huge blue ocean, complete depth, cannot see the bottom. Then there's this huge shape that just starts coming up -- it's like the size of a building!
"And I'm like, I don't even know which way to go right now. It's going to come up and I'm going to be on its back and fall into its mouth or its blowhole or something. I can't see. I am powering with all of my might to try to keep up into the same water space as these animals. It is hard work! You have to be really fit to be able to do what I do."
Mermaids: 'There Is a Big Risk'
What happens next depends upon the animals she approaches, and the ocean's volatility.
"When I was working with the whales, if I had tried too hard to just get in their space they could kind of twitch and a little twitch could completely bowl me over and break my neck," Fraser said. "So there is a big risk."
Fraser is also in the front lines of ocean activism, where danger is just as real above sea level. She was attacked by fishermen while peacefully protesting the annual slaughter of dolphins in Taiji, Japan. The chilling moment was captured in the Academy Award-winning film "The Cove."
"[The fishermen] were extremely angry," she said. "They didn't want to be filmed. And they started backing up their boat propellers right up to our legs. We were that close to spinning boat propellers. And as soon as this guy started pulling out this huge long stick I was like, that's when the fear set in."
The battling forces of culture and conservation continue today. But Fraser's work to defend the earth's oceans as her mermaid alter ego is not solitary. Believe it or not, Fraser is not the only mermaid out there.
Fraser has an underwater colleague in mermaid Linden Wolbert.
"The first time I went underwater and I was out with the fish, looking at the light shining through the kelp, I started to cry," said Wolbert. "It was real. And I dreamed of it my whole life. It was amazing. It was life-changing."
Wolbert's tears of joy at discovering the water led to a new calling.
Like the fictional Aqua Man, Wolbert seems to draw oxygen from the sea. Remarkably, she can hold her breath for nearly five minutes.
"When you do a long breath hold, your diaphragm begins to contract and that's telling you, 'Hey, you're supposed to be breathing,' and you have to learn to tolerate the discomfort of that feeling," said Wolbert. "And once you can achieve that, you can go for quite awhile."
As Wolbert was building her stamina and physical discipline for a life underwater, heartbreak changed her destiny.
"My grandmother ended up becoming sick with cancer," she said. "And the last time I saw her she said, 'Whatever you do, take your gifts and follow your dreams into the ocean.' And I said OK, and she had left me an inheritance. And with that I created my mermaid tail."
Wolbert's dream began to take shape when a remarkable collaborator agreed to build her a one-of-a-kind mermaid tail.
Allan Holt is a special effects artist whose creations have been seen in numerous Hollywood blockbusters.
"When we started this project, my days were [spent] on movies like 'Terminator Salvation' and 'G.I. Joe,' and then I would go and work on the mermaid tail at night," said Holt. "During the days, it's things like carnage and blood. And then at night, it's water-colored paintings of mermaids, and should I have a harp? And should there be gold? What if there's a harp and gold and there's seashell bras? And then the next day it's: How do shotgun shells go through flesh and hit metal underneath and what does that look like? The balance was fun."
Mermaids: 'Immense' Propulsion
After seven months and countless re-workings of the tail, Wolbert and Holt's unique partnership yielded a functional work of art. Wolbert showed it to "20/20" at the Sea Life Aquarium in Carlsbad, Calif.
"It's roughly 35 pounds of silicone. The propulsion I get from this tail is immense. Everything is coming from my core, just like a dolphin," Wolbert said. "So, I am able to really feel the wind through my hair, so to speak, in the water. People can't keep up with me. I can keep up with fish in marine life very easily. I can keep up with the best of them. Being able to swim as fast as a dolphin or close to it is a pretty amazing thing."
It's been two years since the tail's first triumphant splash in the pool. It has traveled on to one of the most spectacular places on the planet: Dean's Blue Hole in the Bahamas -- the world's deepest underwater sink hole.
It's also the site for the Vertical Blue free-diving championships, where Wolbert put her tail aside to judge the competition, and learn technical skills from some of the greatest living divers.
"When I first witnessed people diving deep into the ocean and coming back, I was seeing world champions," Wolbert said. "I was able to really focus on how they moved in the water. I'm able to adopt those different techniques."
And it was here that Wolbert set her personal best record, diving down 91 feet in the 35 pound tail -- and then coming back up again -- on a single breath of air.
But Wolbert also learned that wearing a powerful mermaid appendage can have its challenges.
"You're dealing with a really hydrodynamic piece of equipment," she said. "So when I was just diving down the line, I was getting down there so quickly I actually have to slow myself down."
Wolbert is not just diving to strengthen her already breathtaking abilities. Like a true super hero, she is using her powers for good. The Bahamas has the fourth-highest drowning rate per capita in the world, and Wolbert is on a mission to save its children.
"More than half the people in the Bahamas don't know how to swim, and 100 percent of them are surrounded by water," said Wolbert.
Wolbert saw a way to use the enchanting influence of mermaids for a greater purpose.
"I was with a wonderful non-profit called Swim to Empower," she said. "What their goal is to have every single Bahamian who wants to swim be able to. My focus being on children, I thought, all right, I'm down here for a free-diving competition, I'd love to do something to help this non-profit. So being a mermaid, I wanted to get the kids involved in this program.
"My passion is children. I see children. They see me. And when you have a captive audience, you can send a message. We filmed a public service announcement for anti-drowning awareness and also to help encourage them to swim."
Wolbert has found encouragement from someone she's always admired: Fraser. But, Wolbert had never met her until "20/20" brought them together.
Mermaids are helping children to safely discover the wonders of the sea, and boldly protecting ocean life. Two women who have transcended the line between myth and reality, and have shown there is more to be dreamed of in that realm than ever imagined.
"The magic of having a mermaid do it, it's so unique and it's so different and it's so much fun it's almost like a unicorn trotting up and saying, 'Would you like to learn how to ride horses?'" Wolbert said. "I mean, it's kind of like on that level right? It's something that as a child, I would have been so just completely enchanted by. So I hope that I'm doing that for some other kids."
Watch the full story tonight on "Superhuman!", a special edition of "20/20" at 10 p.m. ET