Even today in the mid-2010s, it’s no exaggeration when scientists say that there are regions of deep space we know more about than the depths of our own planet. So it’s hard to imagine how it must have felt for Jacques Piccard and Don Walsh as they prepared to venture to the deepest point on Earth in 1960. Nobody could be certain what they would encounter and there was a very real chance that this could be a one-way trip.
They were attempting their record-breaking dive in a bathyscaphe, a type of submersible which features an observation cabin which has been specially adapted to survive the stresses of a deep-sea environment (see annotated diagram). And places don’t come much more inhospitable than where the Bathyscaphe Trieste was headed…
Located in the western Pacific, due east of the Philippines, the Mariana Trench is a 2,550-kilometre-long (1500-mile) trough formed as a result of the collision between two tectonic plates. While the scale of the trench is impressive in itself – it’s around 120 times bigger than the Grand Canyon – there’s a smaller valley carved into its base that plunges deeper still. At just under 11 kilometres (seven miles) beneath the ocean’s surface, if you were to pick up Everest and place it in this valley – known as the Challenger Deep – the summit of Earth’s highest mountain would still sit almost two kilometres (1.2 miles) underwater! 
At these mind-boggling depths, it is too far for sunlight to penetrate, so the ocean remains in a state of perpetual darkness and the temperature hovers just above zero. Most concerning of all though is the pressure. At over 1,000 times that at sea level, the success of the dive – not to mention the explorers’ lives – would come down to whether or not the Trieste could take the strain.
The crew embarking on this perilous expedition comprised 28-year-old Navy Lieutenant Don Walsh (USA), an experienced submarine operative, and 37-year-old scientist Jacques Piccard (Switzerland). Along with his father Auguste (himself a record-breaker for flying a balloon to the highest altitude in 1931), Jacques Piccard was the engineering brains behind the Trieste. After seven months of tests and training off nearby Guam, the moment of truth finally arrived on a sticky, overcast day. The date was 23 January 1960.
Things started well, with the bathyscaphe making good headway. As the light began to fade and the temperature dropped with every metre of the descent, Piccard had to carefully release petrol, which served as the Trieste’s means of buoyancy, while Walsh ensured that the ballast (iron pellets) was kept in proportion, to avoid the vessel plummeting too rapidly. 
This was a painstaking balancing act: release too little petrol or too much ballast and they might not make the ocean floor in the limited time they had, but release too much petrol and they may never make it back to the surface. Travelling at a rate of about 0.9 metres (three feet) per second – about the “speed of an elderly elevator”, as Piccard put it – everything was going to plan.
Then came the last thing you want to hear 9,900 metres (32,500 feet) underwater: a cracking noise, accompanied by a jolt. For a moment, they thought they had reached the bottom, but their instruments indicated they were still steadily descending. Could it be the  paint on the cabin flaking, or even a collision with a shoal of shrimp? Unable to determine the cause, but not aware of any imminent danger, the pair decided to continue.  
The next jolt they experienced was much more welcome: they had reached their destination. Sitting on the seabed with 10,911 metres (35,797 feet) of ocean above them, they knew at that moment that they had made history.
It was only when they switched on the lights to get a better look at their surroundings that the cause of the earlier cracking noise was revealed: the viewing port on the entrance tube had fallen victim to the pressure, with a number of cracks crazing the Plexiglas. While this didn’t put the explorers in their sealed sphere in any jeopardy, they realised it could make getting out a lot more tricky if it were to shatter completely, so they decided to cut their allotted time of half an hour on the sea floor short by ten minutes. 
20 minutes might not sound very much after so many years of planning, but it was long enough to answer one of the burning questions on the minds of the entire marine biology world: could any life survive at such extreme depths? According to Walsh and Piccard, the answer is a resounding yes. They report seeing not only shrimp swimming by their viewing window but also a sole-like flatfish, which disturbed by their landing took off from the powdery seabed before disappearing silently into the gloom. It just goes to reaffirm that nature can often beat us humans to a record without even trying!
All that remained was to make the return trip back to the surface. The ballast tanks were dropped, creating a huge, billowing cloud of sediment that briefly swallowed the Trieste, before the vessel rose above it (get a sense for what the sea floor in the Challenger Deep looks like in the video below). 
The rate of the ascent increased exponentially as the rising temperature caused the petrol to expand, up to a top speed of 1.5 metres (five feet) per second. On breaching the surface, just over three hours later, the two new record-breakers received a heroes’ welcome, including several Navy jets swooping low overhead.
To some extent, this incredible voyage went under the radar in the media at the time, due to other groundbreaking feats stealing the headlines. As Walsh would later reflect: “Truth be told, it wasn’t particularly big news. The space race had captured the public’s imagination, and it was much flashier, highly graphic, highly visual, more exciting. We joked that the only thing you see when a sub submerges is a little cloud of bubbles, as opposed to the rockets’ red glare.”
Although two unmanned remotely operated vehicles (ROVs) have ventured to the Challenger Deep since 1960 (one being Nereus in the video above), only one other intrepid hydronaut has followed in Piccard and Walsh’s footsteps: adventure-loving film director, James Cameron. Famed for movies like Avatar (the highest-ever grossing movie) and – more ominously – the ocean-disaster blockbuster, Titanic, he travelled to Earth’s deepest point in his DeepSea Challenger sub in 2012. 
Though he fell about 100 metres (328 feet) short of breaking Piccard and Walsh’s ultimate deep-sea descent, Cameron did claim the deepest solo dive. He also had the opportunity to stay much longer exploring the Challenger Deep than his predecessors – spending around three hours in the abyss, during which he captured the deepest underwater video footage.
On returning to the surface, Cameron described his otherworldly experience: “When I got to the bottom… it was completely featureless and uniform. My feeling was one of complete isolation from all of humanity… More than anything, [it’s] realizing how tiny you are down in this big, vast, black, unknown and unexplored place.” 
Over half a century may have passed since the Trieste’s groundbreaking voyage, but as Cameron’s words attest, there are still countless secrets to uncover – and many records to break – in Earth’s uncharted depths.