Houses fall down. Trees fall over. Large holes form in the ground. Could Alaska be sinking? That's what some villagers in Alaska have been asking themselves recently and scientists think they know why. Warmer temperatures may be causing Alaska's frozen ground to thaw (解冻).

    About 85 percent of Alaska's land surface has permafrost (永久冻结带). Permafrost supports the ground above it—including trees, houses, and roads. But why is the permafrost thawing?

    Some scientists say that human-caused pollution is leading to global warming. And most scientists agree that Alaska has been getting warmer. They say that the warmer temperature is causing permafrost to thaw in some areas. The thawing permafrost can cause problems for people, plants, and animals.

    The collapsing (倒塌) is a problem in the villages in Alaska which are built on permafrost, and some of them may have to move to safer ground, where there is no permafrost.

    The Alaskan town of Glennallen saw its old post office collapse. And, according to James Walters, a permafrost expert at the University of Northern Iowa, house-moving companies have been very busy.

    “Melting permafrost can also destroy trees and forests,” Walters said. “When holes in the ground form, trees fall into them and die. This could turn a forest into a swamp (沼泽), and animals which need the forests will have to move elsewhere.”

    “The melting could cause severe problems.” Walters says, “This could take hundreds of years, but in the end Alaska will look quite different from what it looks like now.”

    Below is a selection from a popular science book.

    If blood is red, why are veins（静脉） blue?

    Actually, veins are not blue at all. They are more of a clear, yellowish color. Although blood looks red when it's outside the body, when it's sitting in the vein near the surface of the skin, it's more of a dark reddish purple color. At the right depth, these blood-filled veins reflect less red light than the surrounding skin, making them look blue by comparison.

    Which works harder, you heart or your brain?

    That kind of depends on whether you're busy thinking or busy exercising. Your heart works up to three times harder during exercise, and shifts enough blood over a lifetime to fill a supertanker. But in the long run, your brain probably tips it, because even when you're sitting still your brain is using twice as much energy as your heart, and it takes four to five times as much blood to feed it.

    Why do teeth fall out, and why don't they grow back in grown-ups?

    Baby (or “milk”) teeth do not last long; they fall out to make bigger room for bigger, stronger adult teeth later on. Adult teeth fall our when they become damaged, decayed and infected by bacteria. Once this second set of teeth has grown in, you're done. When they're gone, they are gone. This is because nature figures you're set for life, and what controls regrowth of your teeth switches off.

    Do old people shrink as they age?

Yes and no. Many people do get shorter as they age. But, when they do, it isn't because they're shrinking all over. They lose height as their spine（脊柱） becomes shorter and more curved due to disuse and the effect of gravity(重力). Many (but not all) men and women do lose height as they get older. Men lose and average of 3-4 cm in height as they age, while women may lose 5 cm or more. If you live to be 200 years old, would you keep shrinking till you were, like 60 cm tall, like a little boy again? No, because old people don't really shrink! It is not that they are growing backwards —- their legs, arms and backbones getting shorter. When they do get shorter, it's because the spine has shortened a little. Or, more often, become more bent and curved.

    Why does spinning make you dizzy（眩晕的）?

    Because your brain gets confused between what you're seeing and what you're feeling. The brain senses that you're spinning using special gravity-and-motion-sensing organs in your inner ear, which work together with your eyes to keep your vision balance stable. But when you suddenly stop spinning the system goes out of control, and your brain thinks you'removing while you're not.

    Where do feelings and emotions come from?

Mostly from an ancient part of the brain called the limbic system. All mammals have this brain area —- from mice to dogs, cats, and humans. So all mammals feel basic emotions like fear, pain and pleasure. But since human feelings also involve other, newer bits of the brain, we feel more complex emotions than any other animal on this planet.

    If exercise wears you out, how can it be good for you?

    Because our bodies adapt to everything we do to them. And as far as your body is concerned, it's “use it, or lose it”! It's not that exercise makes you healthy, it's more that a lack of exercise leaves your body weak and easily affected by disease

    You can tell a lot about a country from its passport. Color is the first give-away. Nations tend to pick colors that reflect their national character. New Zealand decorates its famous rugby team, its airline and its passports in the nation's favorite color—black. Many Islamic countries have green passports because of the importance of the color in the Muslim faith.

    But as well as a statement of national identity, color can show international cooperation. The British passport was originally a navy blue. But after Britain joined the European Union in 1973, it switched to burgundy red（酒红色）like other EU countries. The common color was supposed to make European passports instantly recognizable.

    But since colors can't be patented（专利）, nothing stops others with the same idea. A number of countries that long to join the EU have changed their passports to burgundy too. Meanwhile Britain has decided to leave the club. Its government has announced that the UK passports will return to navy blue after Breit（脱欧）.

    If you have a passport from Singapore or South Korea, you are in possession of one of the most powe3rful travel documents on earth. Passport power is measured by the number of countries the holder can access visa-free. Currently sharing first place in the power rankings, Singaporean and South Korean passports provide visa-free access to 162 countries each. British passport holders share fourth place in the power ranking with six other nations, each accessing 159 countries visa-free. Americans ranked fifth with access to 158. Somalia, Syria, Pakistan and Iraq are among the world's least powerful passports. Afghanistan is the lowest ranking of all, with visa0free access to just 26 countries.

    Having to get a visa is not a piece of cake. First there is the never-ending form. Then there are lengthy appointments and expensive fees. Some visas also require travelers to pre-booked flights to prove that they intend to return home.

    But there are unexpected advantages to some of the world's weaker passports. Middle-ranking Namibia is one of just a handful of countries, whose citizens can enter Angola visa-free. India is one of only three countries, whose citizens have visa-free access to Bhutan.

    A low-ranking passport limits your choices, but not your sense of adventure.

    Chris Thomas used to think of Facebook as just a platform to get in touch with his friends. He now believes the social networking service with saving his life. The 28－year－old was diagnosed with muscular dystrophy(肌肉萎缩症) nine years ago and has relied on a wheelchair ever since.

    On July 15，2015，Thomas was getting ready for bed in his apartment，when a shelving(架子) fell onto his wheelchair and knocked him over，causing him to fall and hit the back of his head. After waking up from a few minutes of coma(昏迷)，he realized he couldn't reach the string used to get the doorkeeper's attention.

    His phone was nearby and had the Facebook application conveniently open，so he posted a status update that immediately got his friends worried—even some he hadn't spoken to in years. “It was amazing，friends from six different states who didn't know each other were connecting with each other to get me help，” Thomas said. “Now that's the power of social media! You have hundreds of people at your fingertips.”

    Eight minutes after the status went up，an old friend，Leah，got in contact with Thomas' friend Brad，whose wife was in the neighborhood，and before you knew it there were 12 Facebook friends in his apartment，who took him to the hospital at once. They showed up to the rescue at the perfect time. Minutes before they arrived，Thomas thought he was going to have a panic attack，which he often suffers from because of his illness，but he's doing better now.

    “It felt good to know that my friends，even those I haven't spoken to in the past，came to save me，” Thomas said. “They all came together .I would've never imagined it.”

    Almost none of us have the time to read everything we'd like to read. Yet we lose countless hours to daily activities that bring us little joy like taking buses and waiting in line. What if we could turn these little blocks of unoccupied time into precious and rewarding moments for learning and thinking?

    Established in 2012, iReader, a micro-learning app on mobile phones, brings the biggest ideas from best-selling books through 15-minute audio (音频) and text. So far, more than 3,000 books have been included, ranging from psychology and parenting to management and economics, with new titles added every day.

    iReader is pioneering a new method of reading, with over 9 million users enjoying the benefits already. According to the Pew Research Center (PRC), the British read just 4 paper books a year and over 25% haven't read a single paper book this year, but reading isn't dying. There are now more ways for the British to read than ever before, due to the widespread use of e-books and audio books.

    The books in iReader are rewritten to ensure it is easy to remember the main content. The way the content is edited has been specifically designed to ensure it is useful in practice. Besides, the content is rewritten with related examples in real life, which means users are more likely to remember and apply what is helpful to them.

    Holger Seim, German co-founder of this app, declares, "iReader gives you the biggest ideas in the shortest possible time. It transforms great ideas into little packs you can listen to or read in just 15 minutes."

Diving in the ocean, marine biologist Erika Woolsey has seen how coral reefs (珊瑚礁) are being damaged by climate change. It has made her decide to find a way to share her experience —including those who can't easily explore the ocean.

Through her non-profit, The Hydrous, Woolsey is using virtual reality to bring the ocean to everyone. Scientists, filmmakers and divers are taking people on immersive (沉浸式的) virtual dives, attracting attention to reef damage and expecting action to protect our sea. About 25% of marine species depend on coral reefs. However, climate change, pollution and overfishing have done harm to around half the world's shallow water coral reefs.

Twenty years of underwater exploration has given Woolsey a detailed understanding of the dangerous situations facing reefs. "I've seen this first-hand shift. Healthy colourful coral reefs become what look like the moonscape step by step," Woolsey says.

It is through this experience that The Hydrous team set out to recreate with their award-winning film Immerse. Intended to watch with a VR headset, viewers join Woolsey for a nine-minute guided virtual div e on the coral reefs, immersed in a 360-degree underwater view.

They swim alongside sea turtles and sharks before witnessing the worsening of the reefs. The experience often brings out strong feelings. "As soon as people take off that headset and look me in the eye, they want to tell me a story about their ocean experience," Woolsey says. "It's that human connection to our ocean that will solve our ocean problems."

Woolsey hopes advances in camera technology will allow her team to take more and more people to places in the ocean that are underexplored and places further away from human civilization. They are developing a virtual experience that will put the people in the role of a marine biologist, carrying out biodiversity surveys underwater, and even transporting the viewers to space to monitor global sea surface temperatures.