Martin Harrell was in good shape when he joined the US Navy’s Sealab programme.

“I was running – not jogging, running – five miles a day,” he says. “I was swimming two miles a day on top of that.”

Despite this, Harrell, already an experienced deep sea diving officer, was by his own admission “nowhere near” the level of physical fitness he would need to attain. One of Sealab’s primary goals was testing, in a high pressure marine environment, the physiological limits of the human body.

Ultimately, the programme would rewrite the rules of what mankind can do below the waves.

Harrell was part of Sealab III, the programme’s third and final expedition in 1969. This month marks the 60th anniversary of when it all began, with Sealab I, on 20 July 1964.

There’s an irony about that date. It was five years to the day before man first walked on the moon. And yet despite this head start, as Research Diving and Training Lead, Phil Short points out, “We still know more about the surface of the moon than we know about our own oceans.”

How long can humans stay under the sea?

DEEP is proud to be sponsoring the upcoming commemoration of Sealab I. On Friday 19 July in Panama City, Florida, the Man in the Sea Museum will host a heroes’ banquet dinner to honour the aquanauts, engineers, and support divers who, six decades ago, made history.

Members of the public can then join the celebrations the following day, Saturday 20 July, at the museum’s Family Day, where they’ll have a chance to get up close to Sealab I, the programme’s first experimental underwater habitat.

It was this painted-red metal chamber, measuring 40ft long by 9ft in diameter, that, 60 years ago, served as an ocean-floor home for aquanauts Lester Anderson, Robert Barth, Sanders Manning, and Robert Thompson. On the surface, 200 feet above, Captain George Bond, a.k.a. “Papa Topside”, was running the mission from a support vessel.

Bond, a qualified medical doctor as well as a Navy diver, had long been interested in the idea of what today is known as “saturation diving”. Traditionally, the amount of time a diver could spend underwater was governed by dive tables, which dictated the amount of decompression time required.

The greater the depth and the more time spent there, the more time a diver would need to spend in a decompression chamber to avoid the bends – the dangerous and potentially fatal condition in which gases absorbed by the body under pressure are released too quickly, forming bubbles in the blood and tissues.

But Bond believed there was a natural upper limit to decompression time. As Officer-in-Charge of the Navy’s Medical Research Laboratory, he spent the late 1950s and early 1960s testing the hypothesis that, for a given depth, the tissues in a diver’s body would eventually absorb all the gases they were going to at that pressure.

The body, in other words, would be saturated. Thereafter, it should be possible to stay at that depth without adding any additional hours to the eventual stay in the decompression chamber.

What do divers breathe at high pressure?

By 1964, Bond and his team had tested the thesis on animals, before moving on to human volunteers. They had learned much about the effects on the human body of prolonged exposure to high pressure.

They had also refined the cocktail of gases the Sealab aquanauts would breathe. The percentage of oxygen, typically around 20% on land, had to be dialled down to around 4%. Any higher and it would be toxic in the high-pressure environment.

They also had to replace some of the nitrogen that makes up most of what we think of as fresh air. Nitrogen produces a narcotic effect at high pressure, so was largely replaced by helium.

There are disadvantages to putting people in a helium environment. For one thing, it makes it very cold. For another, it creates a communication challenge (if you’ve ever heard someone speak after inhaling helium from a balloon, you’ll appreciate why). But, aside from avoiding narcosis, helium has one big advantage over nitrogen: it’s much lighter.

“If you compare drinking a glass of water to drinking a thick milkshake, that’s the difference between breathing air and breathing heliox,” explains DEEP’s Phil Short.

Living and working under the sea

On July 20, 1964, the four aquanauts squeezed into a pressurised capsule. They were then lowered to the sea bed where Sealab I was waiting for them.

The habitat was pressurised to match the water around it, meaning the aquanauts could open a hatch in the bottom and access their new home without the sea following them in. They would then use this hatch to come and go as needed, giving them a base from which they had instant access to the sea at depth.

Bond and his team’s laboratory findings would now be put to the test in a real-world setting.

The mission was slated to last for 21 days. However, it had to be cut short after 11 days when a tropical storm threatened the support vessels providing Sealab I’s air supply. Nonetheless, the experiment proved conclusively that humans can spend extended periods living and working at the bottom of the ocean.

Sealab’s legacy

This conclusion was further established by Sealab II and Sealab III, in 1965 and 1969 respectively, as well as by the Conshelf missions led by French pioneer Jacques Cousteau, also in the 1960s.

In the years since, saturation diving has become a mainstay of the commercial diving industry, allowing divers to reach much greater depths and enabling the installation and repair of underwater equipment and structures such as pipelines.

“Commercial diving took the physiological knowledge gleaned from Sealab and Conshelf and kind of flipped it on its head,” explains Short, “keeping divers under pressure on the surface and sending them down to do jobs.”

Up to now though, humans are yet to establish a permanent presence under the ocean. That idea might sound like science fiction to some, but thanks to the Sealab programme we know it’s not.

Today, DEEP is working to make it a reality, designing modular subsea human habitats and the submarines to get people to and from them.

“What we’re doing at DEEP is going back to that idea of touching the frontier of science,” says Short. “The idea of putting people – photographers, camera operators, scientists, researchers – down there for the entire period of a mission, so they don’t miss anything, and where they can leave the habitat frequently during their working day to get even closer to the marine environment.”

How habitat diving will improve all our lives

Given how much we don’t yet know about the ocean, it’s already the case that divers are finding new marine species on a regular basis. But establishing underwater habitats should speed up this process of discovery.

And as Short explains, the benefits to humanity go beyond scientific curiosity: “A cynic might say ‘Who cares? It’s just another fish.’ But because the marine environment is so difficult to thrive in, those species that do thrive could quite conceivably – and some already have – help to better our own existence, for example through pharmacological research."

Ziconotide, used to treat chronic pain, and trabectedin, an anti-cancer medicine, are two such drugs of marine origin already being used today.

Habitat diving could also play a crucial role in abating climate change. Divers are already planting Posidonia, a type of seagrass, on sea beds around the world. Posidonia has immense potential as a carbon sink, with Posidonia meadows soaking up several times more carbon dioxide than a similar area of rainforest.

The challenge though is that divers have to plant it one stem at a time, and at present they’re limited in how much they can do before they return to the surface. Keeping divers underwater for two or three weeks, with a place to sleep, shower, eat, should vastly accelerate this work.

But the benefits of putting a habitat in the sea will go way beyond ‘known unknowns’ like fighting climate change and finding new medicines.

“We’re going to learn things we never knew we were going to learn without it,” says Short.

That’s why we’re excited to be part of Sealab I’s 60th anniversary this month and to go back to where it all began. To borrow a quote that Sealab aquanaut Martin Harrell used at a previous commemoration: “The difficult we do immediately, the impossible takes a little bit longer.”