Underwater hideouts may be the domain of James Bond villains and Gerry Anderson's Stingray puppets but people in the real world are also dreaming about living at the bottom of the sea - and the dreams may not be far off being realised.
Luxury resorts and restaurants, roaming fleets of research subs and domestic pods with fish-side views are among many ideas floating around, as you might say, for populating the oceans.
The way architect Michael Schutte sees it, with plenty of people prepared to pay a premium for living next to the sea, the next logical step is to start building below the waterline.
"If you've spent $15m on a piece of waterfront property in Miami, what's the next thing that you're going to add to that to actually improve that experience?" he says.
Perhaps build an underwater cocktail bar or a docking station for a submarine to take visitors to see coral reefs, he suggests.
But is anyone, apart from submarine crews, living under water now? The answer is - Yes: the aquanauts.
I put on my diving kit and dropped in on them at the Aquarius Reef Base, run by Florida International University - a research station perched permanently just above the seabed, 20m beneath the waves in the Florida Keys National Marine Sanctuary.
The entrance is a "wet porch". There is no door to knock on, just a shimmering horizontal liquid interface between sea and air. I poke my head through, feeling rather like Alice in Wonderland stepping into the looking glass.
The pressure of air inside maintains this interface with the outside world and holds back the barrage of water that would otherwise flood in.
Once inside, the first thing I notice is my squeaky voice. My vocal cords are having a hard time dealing with air 2.5 times more dense than they're used to.
I ditch my dive kit, take a freshwater shower (salt, which encourages rust, is a menace to the insides of Aquarius), wrap myself in a towel and head straight to a window where I gaze out, mesmerised by the fish drifting through the sunlit waters.
"We have the best view in the world," says Mark Hulsbeck, professional aquanaut and Oceanographic Field Operations Manager for Aquarius, who welcomes me onboard.
"You're inside the aquarium and the fish are watching you."
Within minutes I know it will be a wrench dragging myself away from this magical place.
A base plate weighing more than 100 tonnes stops Aquarius from floating up and a thick cable delivers air, power, wi-fi and mobile phone signal from the surface into the depths.
The living quarters and science lab together occupy a space roughly the size and shape of an American school bus - essentially a long tube which, other than a sphere, is the most efficient shape for withstanding the pressure of 20m of water bearing down on it.
There's a toilet with a shower curtain to pull around for a vague sense of privacy (all the waste from Aquarius is collected and taken back up to land for disposal).
Food, mainly dehydrated camping meals, are prepared in a small galley with boiling water on tap and a microwave. And at the far end are bunks for six people with an escape hatch and spare scuba tanks under the floor in case of emergencies.
I would have loved to stretch out on a bunk and let the fish lull me to sleep, then nip out to dive whenever I wanted to. But after an hour I had to climb back into my dive kit and swim to the surface, just pausing for three minutes on the way, for my body to adapt to the changing pressure.
For aquanauts, leaving Aquarius is more involved. After a few hours under water their bodies have already become accustomed to the compressed air, and they have to prepare very slowly for life above the water.
The hatches are closed and the pressure inside is reduced over the course of 17 hours - any faster and nitrogen bubbles could form in the aquanauts' blood like popping a bottle of champagne. Symptoms of decompression sickness, otherwise known as the bends, can be crippling and lethal.
Once the aquanauts are back at surface pressure, Aquarius is quickly re-pressurised, the wet porch is reopened and the divers can swim back up.
For scientists, living underwater has huge benefits. Aquanauts can dive at 20m and deeper for up to nine hours a day. Surface dwellers can only do it for one hour at a time.
With research facilities like Aquarius, and its predecessor Tektite in the US Virgin Islands, proving that underwater living is feasible, various private individuals are now planning a similar set-up.
One couple from Florida, Dennis and Claudia Chamberland, have their sights set on establishing the first permanent human undersea colony and are taking applications from anyone interested in joining them.
Initially, they plan to build a series of interconnected pods designed along the same lines as Aquarius but instead of having an open wet porch the interior will be kept closer to surface pressure. This removes the need for slow decompression but also reduces the time divers can spend out in the water.
Their idea isn't to be cut off forever but simply for their home address to be "The seabed". People could commute back to land for work, to visit the dentist, pick up groceries and the rest of life's essentials.
- Measures 14m x 3m (46ft x 10ft) and sits on seabed 9km (5.4 nautical miles) offshore
- Houses four scientists and two technicians at a time and is equipped with a kitchen, toilet, hot shower and wi-fi
- More than 350 people saturated inside Aquarius since 1993
It's a lifestyle that Australian adventurer Lloyd Godson has considered. Since 2007 he's been experimenting with various ideas for sustainable underwater living.
In 2010, he lived for two weeks inside a 10-foot box submerged in a public aquarium, breathing air replenished by slimy green colonies of algae and pedalling a bicycle to generate electricity.
For his latest endeavour, with the help of high school students, he is building a solar-powered capsule shaped like a sea urchin. He plans to live inside it for a month while dangling in Darling Harbour in Sydney, Australia.
Godson knows Dennis Chamberland well, and the two men swap ideas and advice about their respective undersea projects.
But Godson admits that he isn't planning on moving his young family to the seabed to become undersea settlers.
"I'm not ready to move underwater long-term," he says.
Among plans architect Michael Schutte has worked on, are a simple steel box with windows for a restaurant off the coast of Vancouver, to an array of six-star hotel rooms destined for warm, clear seas off Fiji.
Another, called H2OME, is the world's first off-the-shelf undersea residence, available to anyone who can secure a piece of seabed to build on.
Much of the necessary technology is available, and becoming more affordable, he says. It includes cell-cast acrylic used to make giant viewing tanks in public aquariums capable of holding back millions of gallons of water.
"There is a special kind of relationship that we have with the ocean or water," Schutte says. "I really believe that there's a future for this stuff."
Listen to The Life Sub-Aquatic on Wednesday 3 September at 11:00 BST on BBC Radio 4 or catch up on iPlayer
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