IELTS Academic Reading Practice Test 61

From the ninth to the fifteenth century, the Khmer Empire ruled much of mainland Southeast Asia. Its capital, Angkor, became one of the largest cities of the ancient world, covering almost one thousand square kilometres. The city contained the famous temple of Angkor Wat, but its survival depended on houses, roads, reservoirs and canals. These were supported by a clever system of water management that coped with the sharp change between the monsoon rains and the dry season.

The climate of the Angkor region is controlled by the southwest monsoon, which brings heavy rain from May to October. For the remaining six months, rainfall is scarce. This uneven pattern made city growth difficult, because flat land could not hold water well. To the south lies the Tonle Sap, a large freshwater lake, whose level rises and falls with the seasons. Khmer engineers therefore built a network of channels, earth banks and storage basins to catch and keep the monsoon water. This made sure water was available through the dry season.

Evidence from inscriptions and archaeological digs shows that water-control structures were built long before the imperial period reached its peak. By the eighth century, they had dug moats around temples and towns, and these ditches served both defensive and ceremonial purposes. As royal authority grew, so did the scale of hydraulic engineering. Successive kings ordered large earth banks and diversion channels that changed the course of natural streams and joined small ponds into one system. This work allowed the city to support far more people than rain-fed farming alone could feed. Inscriptions record these projects as royal acts of merit as well as practical works.

The most striking parts of Angkor's water system are the barays, vast rectangular reservoirs made not by deep digging but by raised earthen dikes. The West Baray was built in the eleventh century. It is about eight kilometres long and two kilometres wide, and can hold roughly fifty million cubic metres of water. An East Baray was built earlier and is a bit smaller, and water enters and leaves through stone gates. Because of their great size, some researchers have described Angkor as a 'hydraulic city', although the exact role of the barays is still open to debate.

A web of canals and smaller pools carried water from the barays to rice fields and households. Other channels supplied the moats around major temples, while others drained extra water back toward the Tonle Sap during floods. In the dry season, stored water could be released slowly to irrigate crops. This system demanded constant care. Sediment carried by flood water collected in the channels and basins. The earthen dikes, which could be worn away, had to be repaired after heavy rains. Keeping the system working meant that the Khmer state regularly called up large numbers of workers.

The water system was not only useful but also tied to Khmer religious and political beliefs. In Hindu and Buddhist cosmology, Mount Meru stands at the centre of the universe with oceans around it. The reservoirs and moats at Angkor recreated this sacred landscape in stone and water. A ruler who built a baray showed his power over both nature and society. A small island temple, West Mebon, built at the centre of the West Baray, reinforced this message.

For much of the twentieth century, archaeologists believed that the barays were mainly agricultural, and that they were used to water rice fields. They reasoned that a city of Angkor's size needed intensive food production. More recent research, including studies of pollen and sediment, has challenged this view. It suggests that some reservoirs may have been kept fairly shallow and free of crops. Much of the city's food was grown on the floodplains rather than on irrigated fields. Some now say the barays supplied water for home use and served as royal monuments, although this revision is still contested.

The gradual abandonment of Angkor during the fourteenth and fifteenth centuries has been linked to environmental stresses, including long droughts punctuated by unusually intense monsoon rains. Such conditions would have damaged canals and dikes and made the water supply less reliable. Although the empire also faced military pressure from neighbouring states, no one cause explains the fall. However, the breakdown of the hydraulic infrastructure appears to have played a significant role in the city's decline.

Modern remote sensing, particularly airborne laser scanning known as lidar, has transformed the study of Angkor's waterworks. It has revealed the remains of canals, roads and settlement mounds hidden beneath dense forest, showing that the engineered landscape was even larger than people had thought. Ground surveys and satellite images have further helped researchers map the flow of water through the old city with great precision. This has turned Angkor into a laboratory for understanding how a pre-industrial society managed water on a regional scale.

Building these huge earthworks required advanced surveying skills and the coordination of thousands of workers. This is shown by the remains of quarries and the huge volume of earth moved. The Khmer state had the administrative capacity to organise such projects, and it extended and changed the water system from time to time. This work reflected both technical knowledge and political power.

A smartphone, a laptop and an electric car may seem to have little in common, but they all depend on the same portable energy source: the rechargeable lithium-ion battery. Since its commercial introduction in the early 1990s, this device has become essential to modern life, powering the mobile electronics that connect billions of people and increasingly propelling vehicles and storing energy from renewable sources. Its contribution was recognised with the Nobel Prize in Chemistry in 2019, awarded to three scientists whose separate discoveries turned a laboratory curiosity into a global technology. Over the past three decades, continued improvements have made the cells smaller, cheaper and more powerful.

The story begins in the 1970s, when the oil crisis prompted researchers to look for alternatives to fossil fuels and for better ways to store electricity. At the oil company Exxon, the British chemist Stanley Whittingham experimented with a cell that used lithium metal as one electrode and titanium disulfide as the other. Lithium was attractive because it is the lightest of all metals and readily gives up electrons, producing a high voltage. However, repeated charging caused needle-like crystals of lithium, known as dendrites, to grow from the metal electrode. These dendrites could pierce the separator between the electrodes and cause short circuits or even fires, making the battery too dangerous for widespread use.

The next major step came in 1980, when the American physicist John Goodenough, working at the University of Oxford, developed a cathode made from lithium cobalt oxide. This material could hold lithium ions between its layers, and it produced roughly twice the voltage of Whittingham's titanium disulfide cell. Equally important, it allowed the battery to be assembled in a discharged state, with no metallic lithium present initially. This meant that lithium ions, rather than lithium metal, would move back and forth during use, a much safer arrangement. Goodenough's cathode is still the basis of many lithium-ion cells produced today.

A practical design still required a suitable negative electrode, and in 1985 Akira Yoshino, working in Japan, replaced metallic lithium with a carbon-based material. The resulting cell used lithium cobalt oxide as the positive electrode and a form of carbon as the negative electrode. Because no lithium metal remained, the dendrite problem was eliminated. Yoshino's design was not only safer but also durable, able to withstand many charge and discharge cycles. In 1991, Sony Corporation combined these ideas into the first commercial lithium-ion battery, and the term 'lithium-ion' was adopted to describe a system in which lithium ions move between the two electrodes without being reduced to metal.

During normal discharge, lithium ions travel from the negative electrode, traditionally made of graphite, through a liquid electrolyte to the positive electrode, which is usually a metal oxide. At the same time, electrons are forced to travel around the outside circuit, providing the electric current that powers a device. When the battery is charged, an external voltage pushes the ions in the opposite direction, storing energy. A thin polymer separator sits between the electrodes, allowing ions to pass while preventing direct electrical contact. This shuttling of ions between the electrodes has led to the description of lithium-ion cells as 'rocking-chair' batteries, because the ions move back and forth without causing major structural changes to the electrodes.

Lithium-ion batteries offer several advantages over older rechargeable technologies. Most importantly, they have a much higher energy density, meaning they can store more energy for a given weight and volume. This property is critical for portable devices and electric vehicles, where reducing mass is a priority. They also exhibit low self-discharge, losing only a few percent of their charge per month when not in use, whereas nickel-based batteries can lose considerably more. In addition, lithium-ion cells have no memory effect, so they do not need to be fully discharged before recharging. These qualities, together with falling costs, have driven rapid adoption in consumer electronics, transport and grid storage.

Despite their success, lithium-ion batteries raise serious environmental and social concerns. The raw materials, particularly lithium, cobalt and nickel, are concentrated in a limited number of regions, and their extraction can damage ecosystems and, in some cases, involve poor working conditions. Cobalt, for example, is largely mined in the Democratic Republic of the Congo, where artisanal mining has been linked to child labour. Recycling rates remain low, and spent batteries contain hazardous materials that must be handled carefully. The thermal runaway that can occur if a cell is damaged or overheated is another unresolved safety risk, although modern battery management systems have greatly reduced the frequency of such events.

Research is now focused on making lithium-ion batteries safer, cheaper and more sustainable. One promising direction is the solid-state battery, which replaces the flammable liquid electrolyte with a solid conductor, potentially increasing energy density and reducing fire risk. Other work aims to reduce or eliminate cobalt, improve recycling, and develop alternatives such as sodium-ion batteries, which use more abundant materials. In the meantime, lithium-ion technology continues to improve incrementally, with manufacturers squeezing more capacity from each cell while lowering production costs. As demand for electric vehicles and renewable energy storage grows, the search for the next generation of batteries has become one of the most active fields in materials science.

Self-driving cars have been promoted as a solution to one of the world's most persistent public health problems: road traffic crashes. Since human error is a factor in the overwhelming majority of collisions, removing the driver from the equation should, in theory, save hundreds of thousands of lives each year. But every incident involving an automated vehicle raises a difficult question that technology cannot answer: when a car drives itself and something goes wrong, who is responsible? My own view is that responsibility cannot simply be transferred to a machine. Humans must remain accountable for the design, regulation and operation of these systems, no matter how much control is handed to software.

The appeal of automation is understandable. Human drivers become tired, distracted, drunk or reckless, and they make errors of judgement that lead to collisions. Autonomous systems, by contrast, never drink or get angry, and their sensors can monitor the environment in all directions simultaneously. Proponents argue that even an imperfect automated vehicle could be safer than a human driver, and that over time the technology will improve. However, this does not mean that autonomous vehicles are infallible. They still encounter ambiguous situations, sensor failures and software bugs, and when they fail, the consequences can be severe.

The legal framework for road traffic assumes that a human driver is in control, and it assigns responsibility accordingly. A driver who causes a collision may face fines, licence penalties or criminal charges. But when an automated vehicle is involved, the human occupant may have done nothing at all. Responsibility then shifts towards the manufacturer, the software developer, the fleet operator or the authority that approved the system. Product liability law becomes central, but it was not designed for systems that change over time through software updates. A car may behave differently after an overnight update than it did when it was sold, raising questions about who should be held accountable for decisions made by code that no single person wrote.

Much public discussion of autonomous vehicles has focused on moral dilemmas, particularly the so-called trolley problem. Engineers are asked how a car should choose between hitting a child or an elderly person, or whether it should sacrifice its own passenger to save pedestrians. I believe this emphasis is misplaced. Such dilemmas are extremely rare, whereas the everyday failures of perception, prediction and control are far more important. A car that mistakes a plastic bag for a rock, or that fails to recognise a cyclist in rain, poses a much greater practical risk than a philosophical puzzle. By concentrating on dramatic hypothetical choices, we ignore the humdrum engineering problems that actually determine safety.

A related problem arises with semi-autonomous systems, which still require a human driver to monitor the road and take over when needed. Research suggests that people are poor at supervising automation for long periods. Attention drifts, and when the system suddenly requests a handover, the driver may need several seconds to regain awareness, by which time a collision may be unavoidable. Some manufacturers have tried to address this with driver-monitoring cameras and alerts, but the fundamental difficulty remains: partial automation can encourage complacency while leaving the human with ultimate responsibility. This gap between expectation and reality is ethically troubling.

When a crash does occur, understanding what went wrong is essential for assigning responsibility. Automated vehicles generate vast amounts of data, but much of it is stored in formats that are difficult for outsiders to interpret, and the algorithms that make decisions are often protected as trade secrets. For a fair legal process, investigators and courts need access to reliable records and explanations. This is not only a technical issue; it is a matter of justice. If the reasons for a crash cannot be explained, victims and their families may be unable to obtain redress, and regulators may be unable to prevent similar incidents.

Regulation is currently fragmented. Different countries, and even different regions within the same country, have adopted different rules for testing and deployment. Some jurisdictions allow fully driverless vehicles on public roads, while others require a safety driver at all times. This patchwork creates uncertainty for manufacturers and the public alike. A more coherent approach would establish common safety standards, mandatory incident reporting and independent oversight. Without such measures, the race to bring autonomous vehicles to market may outpace the development of rules to govern them.

I do not argue that autonomous vehicles should be abandoned. They have the potential to reduce road deaths and improve mobility for people who cannot drive. But responsibility must be distributed realistically across all those involved: engineers, companies, regulators and users. The algorithm is not a moral agent. It cannot be blamed, punished or held to account. Only humans can bear responsibility, and any legal system that pretends otherwise will fail both victims and the wider public. If we are to accept automation on our roads, we must also insist on transparency, rigorous testing and clear lines of accountability.

Public acceptance of self-driving cars will depend on more than technical performance. People need to believe that, if something goes wrong, a fair and understandable process will determine what happened and who should answer for it. Surveys repeatedly show that many people remain cautious about riding in a vehicle without a human driver, and a single well-publicised crash can set back confidence for years. Building trust will require not only safer technology but also a legal and regulatory framework that treats accountability as seriously as innovation.