Luyện theo cấu trúc Cambridge IELTS 4 · Listening Part 4: Exploring the Deep Ocean

Lecturer: Good morning, everyone. In today's lecture in our marine science series, I'm going to talk about how scientists explore the deep ocean, that is, the part of the sea below about two hundred metres, where sunlight can no longer reach. Although this zone makes up the vast majority of the living space on our planet, we still know surprisingly little about it. In fact, it's often said that we have better maps of the surface of the Moon than of our own sea floor.

Let's begin with the conditions down there, because they explain why exploration is so difficult. The first problem is pressure. For every ten metres you descend, the pressure increases by roughly one atmosphere, so at four thousand metres, which is about the average depth of the ocean, any vehicle has to withstand around four hundred times the pressure at the surface. The second problem is darkness. Below a thousand metres there is no sunlight at all, and the only light comes from the animals themselves. The third is temperature: the water near the bottom of the ocean is typically just two or three degrees above freezing.

So how do scientists actually get down there? Early researchers relied on lowering nets and dredges from ships and seeing what came back up. This told them that life existed at great depths, but the animals were usually damaged on the way up, and of course the researchers could see nothing of how those creatures behaved. The next stage was crewed submersibles, small vessels that carry two or three people. These allowed direct observation for the first time, but they're expensive to operate, and a single dive can take around six hours. No, sorry, I've got that wrong. It's closer to eight hours, and much of that time is spent simply going down and coming back up.

Today, most deep-sea work is done by robots, and there are two main types. The first is the remotely operated vehicle, which is connected to a ship by a long cable. This cable carries power and data, so the pilots on the ship can watch live video and control robotic arms to collect samples. The second type is the autonomous vehicle, which has no cable at all. It follows a route that has been programmed in advance, and it's mainly used for mapping, because it can cover large areas of sea floor while the ship gets on with other work. The main limitation of autonomous vehicles is the battery, which generally lasts for less than two days.

Now, what have these technologies revealed? One of the most important discoveries was hydrothermal vents, openings in the sea floor where very hot, mineral-rich water comes out. Scientists were astonished to find thriving communities of animals around them, including giant tube worms. These animals don't depend on sunlight at all. Instead, bacteria convert chemicals from the vents into energy, and the larger animals feed on these bacteria or live alongside them. This changed our ideas about where life could exist.

Another major area of research is bioluminescence, the production of light by living things. It's been estimated that around three-quarters of deep-sea animals can produce light. They use it for several purposes: to attract prey, to find a partner, and, perhaps surprisingly, as camouflage. Some fish produce a faint glow on their underside which matches the dim light coming from above, so a predator looking up can't see their outline.

Finally, I want to mention some concerns about the future. Several companies are interested in mining the deep sea floor for metals that are used in electric car batteries. Supporters argue that this could reduce the need for mining on land. However, many biologists worry that mining would destroy habitats that took thousands of years to form, and that the clouds of sediment stirred up by the machines could spread for many kilometres. The real problem is that we simply don't have enough baseline data, that is, information about what these places are like before any disturbance, to predict the impact with any confidence. And that brings me to your assignment for next week.