[0:01]Earth, a 4.5 billion year old planet, still evolving. As continents shift and clash, volcanoes erupt, and glaciers grow and recede. The Earth's crust is carved in numerous and fascinating ways, leaving a trail of geological mysteries behind. In this episode, the Mariana's trench, the deepest point on Earth, is explored. Its sheer walls cut seven miles into the Pacific Ocean. The mystery of what created this deep dark chasm takes science detectives on some of the most dangerous dives ever attempted. Deep into the abyss, scouring the ocean floor, scientists uncover a strange undersea world of fiery mountains, bizarre mud volcanoes, and the largest geological structure on Earth. Discoveries from this unique underwater world will revolutionize our understanding of the powerful forces that shape not just the trench, but the Earth itself.
[1:25]Hidden deep beneath the waves of the Western Pacific lies the Mariana's trench, the deepest point of all the oceans. The first step on the journey of what created this mysterious scar in the Earth's crust and how it continues to mold the planet, takes us back to 1872. When a British research vessel HMS Challenger, set out on the first ever mission to map the ocean floor. Throughout most of recorded history, men have just assumed that beyond a certain level, the sea was pretty flat, pretty dead, fairly lifeless.
[2:07]They weren't expecting to find anything very interesting. For four years, the Challenger crisscrossed the oceans, covering 70,000 miles, a third of the distance to the moon. The crew plumbed the depths every 140 miles, using a total of 249 miles of rope and hundreds of pounds of lead weight. It was tedious, back breaking work, but at the time it was the only way to measure the depth of the ocean floor.
[2:46]When they got to the Western Pacific, 200 miles off the island of Guam, the crew routinely lowered the rope for a measurement.
[2:59]But the weight kept on dropping and dropping. It's a big surprise. Nobody thought the ocean was this deep. So, all of a sudden we've got scientists saying, why is that?
[3:20]Eventually, the weight struck the bottom at 4,475 fathoms, nearly five miles beneath the ocean surface. The scientists would be going, wow, we found something and what does it mean? Is it a little hole? Is it a big hole? What kind of feature is it down there? There's a whole lot of questions you get when you find this one spectacular reading.
[3:47]The Challenger expedition marked the birth of modern oceanography and provided the first crude map of the ocean floor.
[3:56]It showed how the ocean floor gently slopes away from the land and then plummets thousands of feet into vast flat plains. But the Western Pacific is different. It drops off again into the five-mile deep hole. A hole that blew right out of the water the long held belief that the sea floor was flat and featureless.
[4:21]And it spawned a mystery, because nobody could understand how this strange underwater feature came about. It would be 75 years before any answers emerged. It took a revolutionary new technology, Sonar, to push the investigation forward to the next crucial stage.
[4:48]Sonar was first developed in the early 1900s and then perfected during the 1940s to detect submarines lurking in the deep.
[5:02]The system works by pumping sound waves through the water. The waves bounce off solid objects and are reflected back to a detector. By measuring the time it takes for the sound waves to bounce back, scientists realized they could build a remarkably accurate picture of the world beneath the waves. The world's major navies spent a lot of time and effort developing submarine hunting technology. Then the hydrographers discover that you can use this to chart the bottom of the sea, and it's an awful lot cheaper and easier than using large numbers of sailors pulling on ropes.
[5:42]In 1951, a British Navy research ship returned to the deep hole found by the Challenger expedition. But this time, they were armed with sophisticated new sonar equipment, and the results were amazing.
[6:04]Detailed sonar maps revealed that the deep hole in the Pacific Ocean floor isn't a hole at all, but part of a massive trench, 30 times deeper than the Empire State Building is high.
[6:18]It runs twice the length of California, 1500 miles from the southeast of Guam to the northwest of the Mariana Islands.
[6:31]People were probably astounded by what they were seeing because clearly the ocean floor had enormous changes in relief, was very mountainous in some places, had great deeps in other places. To a geologist, this would be extremely exciting. Even within the trench itself, there are remarkable variations. At its southern end lies the greatest surprise of all. The sea floor drops down another two miles to its lowest point, a staggering seven miles beneath the waves.
[7:08]Scientists had discovered the deepest part of the oceans. Even today, it is the lowest known point on the planet.
[7:20]They named this part of the trench, the Challenger Deep, in honor of the ship that discovered it.
[7:28]To get a sense of just how deep trenches are. If we take the height of Mount Everest, we would still have about a mile of water above us before we get to the ocean surface.
[7:38]But how the Mariana's trench was formed, remained a mystery. Investigators decided the best way to find the answer was to dive to the bottom of the trench, to see for themselves the lowest point on the planet, the Challenger Deep. But they faced a major problem. At the bottom of the trench, they would have to contend with pressure a thousand times stronger than at the surface.
[8:09]That's the equivalent of being squeezed on all sides by the weight of 50 jumbo jets.
[8:17]To demonstrate the effects of such pressure, scientists use a dummy head. Today what we're going to do is actually put one of these Styrofoam wig heads in the pressure chamber.
[8:33]And expose it to the pressure we would see in the Mariana's trench. That's about 16,000 PSI. A human skull would be crushed to a pulp, but the rubbery head will only have all the air squeezed out. Wow! That's smaller. And here's what the original size was. Just for comparison, quite dramatic. Pretty stark difference between something that hasn't been seven miles deep in the ocean and something that has. Glad I'm not going there.
[9:11]At the Mariana's trench, human life is impossible. We're not equipped to resist those kinds of pressures. And so it's necessary to protect humans from that type of an environment. The challenge to engineers was how to accomplish this. In 1953, Swiss scientist August Piccard designed the Trieste, a pioneering vehicle that could withstand the crushing pressures.
[9:46]The submersible was dominated by a 50 foot long hull, filled with light aviation gasoline and lead weights to control buoyancy. Slung underneath it was a tiny, six-foot spherical cabin with five-inch thick steel walls.
[10:07]Finally, after seven years of modifications and manned test dives no deeper than three and a half miles, the Trieste was ready to attempt the seven miles to the bottom of the trench.
[10:20]The commander of this perilous undertaking was US Navy Lieutenant and deep sea explorer Don Walsh. I know the astronauts that go through this all the time. Why do you have to be there? Why can't we just put up a robot to do things? Got to be there because that's what we do. Only a few officers and scientists knew about the risky mission, which was launched in January 1960 from the Western Pacific island of Guam. Guam in those days was kind of a backwater. It was just right for us because we were trying to do this project sort of out of sight because we weren't too sure it was going to work. The Navy just didn't want to be embarrassed by a failed science spectacular.
[11:08]Accompanying Walsh was the son of the Trieste designer, engineer and oceanographer Jacques Piccard. The two men would spend the next nine hours squeezed inside the cramped sphere. And we had, um, 20 cubic feet of space inside. That's about the same as a household refrigerator. And the temperature was almost that cold inside. It was a drama.
[11:37]The story of how the Mariana's trench came to be is beginning to take shape. In 1874, British surveyors were the first to discover a five-mile deep hole in the ocean. 75 years later, sonar mapping revealed the hole to be a vast, 1500-mile long trench, with the deepest part seven miles beneath the surface waves of the Pacific. To gather further evidence, two courageous men were about to undertake the most dangerous dive in history. They would venture into the abyss and go to the bottom of the Mariana's trench. The Mariana's trench is one of the most remote, inhospitable places on Earth.
[12:30]In January 1960, two deep sea explorers, Don Walsh and Jacques Piccard, plunged into its depths on board the submersible, the Trieste.
[12:47]At a speed of just three miles per hour, they began their slow descent into the twilight zone.
[12:56]By 3,000 feet, the darkness was total. The only illumination was from the Trieste's powerful lights.
[13:08]The depths we were operating at, it was always black. The only thing that lit up the abyss was the bioluminescence from animals and plankton.
[13:20]Like fireflies, they carry their own light sources with them. Encased in their five-inch thick steel sphere, Walsh and Piccard quickly passed their test dive record of 18,000 feet. Everything appeared to be going to plan.
[13:38]At the rear of the cabin, the crew were protected by a double layer of glass, but two hours into the dive, the outer pane cracked.
[13:50]We, um, had a great big bang. We didn't know what it was. We're about 20,000 feet, and we looked around and checked everything. Every square inch of their tiny life-supporting capsule was fighting back eight tons of pressure. With the outer pane broken, the only thing between the men and instant death was a single pane of glass. If the inner window had cracked, we would have been instantly dead. Maybe even before we knew it. But incredibly, the inner pane remained watertight. Walsh and Piccard decided to continue the descent. After a tense, claustrophobic four hours and 48 minutes, they approached the bottom of the trench, only to be startled by movement on the sea floor. Just before we landed, we saw a flatfish about a foot long.
[14:51]And that's a bottom dwelling fish, so if you see one there are others. Nobody expected to see life at these crushing depths, but it meant the explorers had reached their goal.
[15:05]The very bottom of the Mariana's trench. The depth gauge with a reading of 35,800 feet, nearly seven miles below the surface, confirmed the sonar findings.
[15:24]Squeezed inside their bubble of breathable air, the two explorers were closer to the Earth's center than man had ever been. We took a self portrait. That's the picture that you see. We said we're gonna do it and we did it.
[15:43]But there was work to be done. Walsh and Piccard wanted to make detailed observations of the enormous trench.
[15:53]Unfortunately, the Trieste stirred up a cloud of fine, powdery sediment from the sea floor that obscured their view. It's like being in a bowl of milk at that point, so realizing we weren't going to see anything, we decided to go on back up to the surface. Off the island of Guam, the Trieste surfaces after a descent into the Mariana's Trench. After nine grueling hours underwater, Walsh and Piccard returned to the surface on January 23rd, 1960. And officially entered the record books for the deepest dive of all time. To this date, their extraordinary feat has never been repeated.
[16:37]The mission was a success, but the mystery remained. Geologists still didn't understand what could have formed the immense trench. And if they couldn't find the answer inside the trench, they would have to look elsewhere.
[16:58]Perhaps there was something somewhere on the ocean floor that might explain the trench's origins.
[17:09]Throughout the 50s and 60s, a team of geologists led by Princeton's Harry Hess compiled sonar data from all of the world's oceans. It was as though they had pulled out a giant plug to drain away all the water and expose the ocean floor. Their maps revealed that the Mariana's trench is just a tiny fraction of a network of enormous underwater canyons stretching right around the planet. But that wasn't all. Running parallel to the trench on the other side of the Pacific, the maps showed a giant underwater mountain range, the East Pacific Ridge. And this too is part of a global network. A 40,000-mile long chain of mountain ranges that ring the globe like the seams of a baseball to make the largest geological feature on Earth.
[19:57]Studying the ocean ridges led geologists to believe that magma welling up at the ridges was pushing the plates apart. But this presents scientists with a puzzle. If new crust is being created at the ocean ridge and the Earth isn't expanding, then the old crust must be disappearing somewhere else.
[20:47]The plates at the trenches are sinking down into the mantle and pulling the plates apart, at the ridges, and the magma just passively fills in the gaps.
