THIRTY YEARS AGO, there were 50 or so underwater habitats in the world's oceans, but almost all of them were soon abandoned to rust quietly on the bottom.
Jacques Cousteau and others believed that by the end of the 20th century the continental shelf would be littered with pressurised bungalows from which residents would emerge to excavate wrecks, or scoop up diamonds and manganese nodules. For relaxation, they would clear allotments of seaweeds, or ranch fish.
But Wetropolis never came to pass. To sustain ourselves under water is a costly business, requiring substantial logistical support from the surface.
Saturation diving has been widely used in deep-sea operations, but the divers haven't lived on the bottom. We can descend into the deeps, but only
if encased in a watertight submersible, or kitted out like a gasworks on legs.
Yet we have an evolutionary affinity with the sea.
Why else would we put so much salt on fish and chips, or flock to the seaside every summer to get braised pink?
What is more, like the world we are seven-tenths salty water, and when still in the womb we even had gill slits.
In 1960 a distinguished zoologist, Sir Alister Hardy, spiced up his talk at a BSAC conference by proposing that human beings had an amphibious phase in their evolution. He claimed that we still bear telltale tokens of our aquatic ancestry: webbed digits are a common birth "defect", our hair follicles follow the flow lines of water over the body, and very young babies have no fear of being immersed and instinctively hold their breath and swim - as every Nirvana fan knows.
When diving mammals are under water, their blood supply is allocated to the most oxygen-sensitive organs, such as the heart and brain, at the expense of other tissues. This also allows the heart to slow down, for now it's supplying a much smaller circuit.
This reduction in physiological activity cuts oxygen consumption and prolongs a dive. Much has been made of the fact that we too possess this "diving reflex", but in humans it's merely a token response. Even in experienced freedivers, the heart rate is reduced only by about 45%.
Experiments on divers demonstrate that this reflex is too weak to reduce significantly the body's oxygen consumption. In any case, it takes about 40 seconds for the full effect to develop, by which time the average breath-holding dive for most of us would be almost over. If the diver breathes in before diving, it slows the development of the effect even more.
David Blaine's recent attempt to hold his breath for 9 minutes fell well short of the target. Freedivers are adept at apnea (being without breath) and occasionally permapnea (never breathing again), but even they just exceed eight minutes while floating motionless face-down.
Yet the need to breathe air is clearly not a major impediment to living an aquatic life. Of course, it helps if you can hold your breath for over an hour, like the sperm whale and the bigger seals.
It was thought that the Fitzroy turtle could hold its breath for three days, until it was caught cheating by breathing through its bum.
Diving animals are so skilled at breath-holding that they even do it for no reason. A seal dozing on the beach often stops breathing for long periods. Could it be to shut out the smell of the colony?
Diving mammals store oxygen much as a desert cactus stores water. An elephant seal has five times more stored oxygen than a human. But there is no point using the lungs as a reservoir, because as the animal descends its lungs collapse when the air inside is squeezed. Twenty metres down, the lungs are reduced to only a 20th of their original volume, and on really deep dives they become a solid lump. Instead, the
deep-diving seals and whales store at least half of their oxygen in the blood.
Big animals have an enormous advantage - an elephant seal has three times more blood than we do.
Bigger bodies also have relatively more "inert" tissue such as fat and bone, and proportionally less oxygen-hungry tissue such as muscle.
So, with increasing size, the ability to store oxygen grows faster than the body's demand for it, which really helps to extend the duration of dives.
However, size, as some divers will tell you, isn't everything. Little guillemots are accomplished underwater swimmers and have been filmed playing around 90m down.
Diving mammals also store up to a third of their oxygen in the myoglobin of their muscles. Myoglobin is what makes meat red, but there is so much in whale and seal muscle that it's purplish-black. An elephant seal has 10 times more myoglobin than we have.
For most freedivers, breath-holding is a means to break depth records and then, ideally, to return to the surface.
However, the emperor penguin, which can hold its breath only twice as long as a freediver, can descend three times further (500m), and shuns the advantages of a weighted sled on the way down and a balloon on the way back.
The modest elephant seal never boasts that it can dive down over a kilometre, nor does the sperm whale that reaches 2km.
Research on bottlenosed whales that regularly descended to 800m every 80 minutes or so revealed that they don't get out of breath. They take no longer to recover from a prolonged dive than from a short one.
In contrast, at a World Freediving Championship every contestant surfaced close to asphyxiation, 15 blacked out and one had a heart attack.
Since its birth, freediving has continually confounded the medics' warnings of what the human body could withstand.
Only 40 years ago, they declared that the lungs could not cope with the pressure 50m down - a depth now routinely used for warm-up dives.
But the limits must be getting dangerously close. Claes Lundgren, a physiologist who studies free divers, warns that: "No researcher would say it's safe to go so deep. It's a very fine line they are treading at those depths.
"If they really knew what they were putting their bodies through, I don't think they would dive. We are not meant to be deep divers."
We are not even great swimmers.
The yellowfin tuna can cover 100m three times faster than a human sprinter can on land. The most unlikely animals are stronger swimmers than we are. The record distance for elephants when they go snorkelling with their trunks held high is 48km.
Another problem is that we feel the cold, and water conducts warmth away from the body 25 times faster than air.
Diving marine mammals never skimp on insulation. There is no such thing as a skinny dugong or an anorexic whale. The blue whale is lined with blubber up to 50cm thick, constituting almost half its body weight. Blubber makes marine mammals so buoyant that most don't take a big breath before diving - they breathe out and half-empty their lungs.
We are the plumpest primates and the only ones with chubby babies, but the thin fat layer coating an athletic person is hopelessly inadequate insulation in water.
If you fall overboard from a ship, you will probably die from hypothermia before drowning. When the Laconia sank in the warm Mediterranean in summer, 200 passengers floating on the surface died from hypothermia.
Japanese Ama divers subject themselves to a greater cold stress than any other group of human beings. They lose around 600 kilocalories during a single diving shift in winter, and their deep body temperature drops by well over 2°C. They consume half as many calories again as their non-diving sisters, yet half the Ama's body fat is lost in winter as she lives off her reserves.
We are not just frigid, we're also blind, for without a dive mask our underwater vision is hopelessly blurred.
However, the Moken people of Thailand live entirely by fishing and diving without a mask or goggles.
Tests have shown that although on land their eyesight is no better than that of Europeans, under water it is vastly superior, even in children.
The normal response of the pupil is to dilate in the dimmer light below, but the Moken contract their pupils, which sharpens the focus, just as seals do to compensate for their natural short-sightedness. So perhaps we can learn to see better under water.
Even if our original ape ancestors were never truly aquatic, many early humans were indeed amphibious, and this may have had major consequences for our evolution.
The human brain showed a sudden growth spurt during the last 200,000 years, with a 50% increase in cranial capacity from Homo erectus to us, Homo sapiens.
The fatty acids that stimulate brain growth are scarce on land, but abundant in aquatic creatures.
Many ancient human settlements were beside the sea, as indicated by huge shell middens, the remnants of a thousand meals. Coastal sites in South Africa and on the Red Sea reveal that 100,000 years ago the main food of Homo sapiens was shellfish.
This was a recipe for brain expansion. Even my granny knew it: "Eat up your fish, Trevor. It's brain food."
Anthropologists are at last recognising that many of the differences between humans and chimpanzees, with which we share around 99% of our genes, are explained by a waterside past.
We were fishers and sailors and, in spite of our physiological inadequacies, perhaps even divers long before we were farmers.
There is no such thing as an anorexic pilot whale
An under-water habitat - but being truly self-sustaining is difficult.
Elephant seals can dive more than 1000m deep
The Moken people of Thailand have amazing eyesight under water - including the children
Yellow-fin tuna can travel three times faster than a human sprinter
Freedivers have dived beyond 200m on a single breath of air
TREVOR NORTON is the author of Stars Beneath The Sea and Reflections on a Summer Sea, and his latest book, based on his own life, is Under Water To Get Out Of The Rain, which tells the story of how his love for the sea developed from boyhood family holidays in Whitley Bay to his first attempts at diving and beyond. The hardback book is published by Century (ISBN 0712638849) and costs £12.99.
Trevor, an authority on the history of scientific diving, is Professor of Marine Biology at the University of Liverpool and Director of the Port Erin Marine Laboratory on the Isle of Man.