THERE’S not a lot of elbow room inside the Limiting Factor, a titanium sphere that has been at the heart of Five Deeps, an expedition to visit the deepest points of each of the five oceans of the world.

Victor Vescovo, the Texan who funded it and has been the test pilot and solo driver on its “dive firsts”, is sitting inside it as it rests on a boat berthed in Edinburgh – and it’s tight.

Vescovo is just the fourth person to have dived the 10,928 metres of Challenger Deep to the bottom of the ocean – following in the wake of Don Walsh and Jacques Piccard in 1960 and film director James Cameron in 2012. He also happens to be the first to have visited both there and Everest’s summit. He has already done over a 100 hours of diving in this ball.

Most people who know subs, he says, tend to tell him it’s bigger than they expected. But it’s still cramped. It contains no bathroom – the toilet being a bottle. For windows it has three small, fish-eye portals. The hull walls, designed by pioneering engineers Triton Submarines, are 9cm thick – and need to be. As 53-year-old Vescovo puts it: “If you put three aircraft carriers on top of the Limiting Factor, that’s how much weight is surrounding it. It’s eight tonnes per square inch.”

Not that he’s aware, he says, of that pressure when he is falling through the water. “After the first 10 metres, it’s so peaceful. You can’t even really tell you’re moving, unless you look out of the portals and see the bubbles or fish. It feels like you’re just sitting in a chair. However, your mind is watching that depth gauge and intellectually, you’re thinking this is getting really deep. With something like the Challenger Deep when you’re going 9,000, 10,000 metres, you’re thinking, ‘There’s a lot of pressure out there and if it cracks I’m not going to be here.’”

On his Challenger Deep dive he took the ice axe that he used at the peak of Everest as company. “It was extraordinary to be at the bottom of the ocean,” he recalls, “and only three other people had been there before. I spoke with James Cameron beforehand and he said, ‘Make sure to turn your thrusters off, and sit and appreciate where you are.’ That’s what I did. I literally just sat back in my chair, ate a tuna fish sandwich and just drifted on the bottom of the ocean for about 10 minutes.”

But the risks are clearly there. Above his head is a battery of oxygen canisters – enough, Vescovo points out, to last him four days should anything go wrong. But he says his worst-case scenario plan would be to just turn the oxygen off and go to sleep. “Not a bad way to go.”

What’s groundbreaking about Five Deeps is that it has demonstrated that the Limiting Factor is a vessel that can be used again and again. Neither Walsh and Piccard’s Trieste nor Cameron’s Deepsea Challenger were able to dive repeatedly like this. As Kelvin Murray, a Scot from EYOS Expeditions which is managing the project, observes: “There was a point when we were diving in the Mariana trench repeatedly and it became ordinary. It was like we now have an elevator that goes to the bottom of the ocean. To have that kind of technology is important. We know so little about what goes on in our deep ocean, yet these things are massive drivers for our planet. It’s like we have been given the keys to find out.”

Among the big news stories to come out of the expedition so far has been the fact that Vescovo saw what looked like plastic at the bottom of the Mariana trench. However, for the explorer himself, the story is much bigger, the real revelation being how little we still know about the workings of the ocean. “So many people are so concerned about how we’re impacting our world, and yet 70% of it is driven by this water column that we still know very little about.”

A former navy officer and private equity investor, Vescovo has been compared to Elon Musk in being an entrepreneur who is going where governments aren’t in their development of exploration technology. In a TED Talk he even joked: “[Elon], I’ll give you a ride in mine, if you’ll give me a ride in yours.”

He says: “SpaceX have been developing reusable rockets – can one imagine Nasa in the same timeline doing that? – and we’re an analogue to what Musk and Jeff Bezos are doing with reusable rocketry. We’ve developed a reliable, reusable deep ocean submarine.”

Right now the submersible is sitting on the deck of the mission mothership, the Pressure Drop, berthed outside Edinburgh’s Ocean Terminal. For some on board this is like coming home – since many involved in the expedition are Scots. Among them, for instance, is chief scientist, Dr Alan Jamieson, the first British person to have gone to the hadal zone, below 6,000 metres, and therefore the “deepest Scot” – though not the “deepest Briton”, since John Ramsay, the English designer of the sub, has since gone deeper.

Jamieson has been on the project from its early days. Originally an industrial designer, he began his journey to becoming a top hadal zone expert when he developed an interest in why people weren’t building technology to go deeper than 6000 metres. He realised it was possible and, he recalls: “One thing led to another and the turning point was in 2008 when I filmed tonnes of fish at close to 8,000 metres depth just off Japan. It went completely viral. The team here brought me on board because I wrote a book called The Hadal Zone. They were just like, ‘Who wrote this book? Get me this guy!’”

His own 12-hour dive down into the Java trench wasn’t something that had been initially part of the plan. Rather, it was “dropped” on him by Vescovo over dinner one night. “We targeted a wall at about 7,500 metres depth. The idea was to go somewhere you couldn’t normally drop a camera onto. At one point we ended up in what was a cave but didn’t realise – there were sponges hanging on this underhang, like bats.”

Jamieson had never done a submersible dive before. “I like the fact that the first time I ever got in a sub was to 7,000 metres.”

Scientifically the trip has been significant in terms of finding over 40 new species, and mapping the floor. Working alongside Jamieson is Scottish geologist Heather Stewart, of the British Geological Society, who describes one of the joys of Five Deeps as being “the opportunity to do some blue skies science”. “This was proper exploration. These are places that there was very little data about.”

Like Jamieson, she did a dive. She recalls: “It was truly amazing. You’re free-falling through the ocean. You see the colour change from the greens on the sea surface, and then you’re coming up to the seabed and it was breathtaking seeing that emerge.”

Meanwhile, other key Scots on the team include the Pressure Drop’s captain, Stuart Buckle, who also captained James Cameron’s expedition. “I had market monopoly on sending people down to the deepest point because I’d done James Cameron,” he says. “And that happened by pure chance. My ship was in Thailand waiting for the next job and the ship broker came down and said we would like to launch a submersible off your deck, can you do it?” Not to mention, Kelvin Murray, whose team at EYOS is the onshore “glue” holding the project together.

Along the jamb of the doorway to the Pressure Drop’s lounge is a series of pen marks which Vescovo points out. They look very much like the height markings people often place around doors to chart a child’s growth, only it turns out they are depths, representations of where they have been – the top of the door the surface, the bottom Challenger Deep. He points to 4,000 metres, the depth he was at in the Southern Ocean, when he had a communications outage. “No submarine had ever dived in the Southern Ocean to any reasonable depth,” he says. “So I felt like an astronaut, going where no-one had ever gone before. But I was totally fine with it because I fly a lot, fly planes and I’ve had radio outages before.”

As it happens, also in the lounge is one of the few men who knows what it’s like to make such a pioneering descent, 87-year-old Don Walsh, one of the first to Challenger Deep. Was he jealous when he saw Vescovo doing his own dive there? “No,” he says. “My feeling is that they should have been doing this 55 years ago. When, in 1960, we surfaced, Jacques Piccard and I were sitting on top, and we said ‘How long till somebody gets back here?’ We finally agreed that two years would be about right. We thought that was very conservative.”

Vescovo describes the development of the Limiting Factor as the “reopening of a door” – one that Walsh opened. “What we are hoping this does is reignite the passion for deep ocean exploration. I like to quote Winston Churchill: ‘This is not the end. It is not even the beginning of the end. But it is, perhaps, the end of the beginning.’”

The Five Deeps

Pacific Ocean: Challenger Deep

In the Mariana Trench, the deepest point on Planet Earth. Vescovo dived 10,928 metres - deeper than any previous 10,912 metres dive by pioneers Don Walsh and Jacques Piccard in 1960 in the Trieste or James Cameron's 10,908 metres in Deepsea Challenger in 2012.

Atlantic Ocean: Puerto Rico Trench

The deepest point in the Atlantic Ocean – the verified bottom of the Puerto Rico Trench at 8,376 metres.

Indian Ocean: Java Trench

The deepest point of the Java Trench in the Indian Ocean has now been measured at 7,192 metres and Vescovo was the first human to dive to its depths.

Southern Ocean: South Sandwich Trench

In the southern portion of the South Sandwich Trench the deepest point is 7,433.6 metres. Waters in the hadal zone of this trench are at subzero temperatures. No person, prior to Vescovo, had ever dived in this trench, let alone got to the bottom.

Arctic Ocean: Molloy Trench

The expedition's final mission, and least deep dive at around 5669 metres.