4 . Mammoth Cave National Park, in the state of Kentucky, is a unique and historical natural wonder. It has the world’s longest cave system. There may be another 600 miles of cave passageways that have yet to be explored. In addition, more than 200 caves that are disconnected from the larger system can be found in the park.

The underground caves were created naturally by the process of limestone (石灰岩) erosion. Rain and rivers slowly dissolve (溶解) and shape soft limestone. The system is still being shaped today. Not only are they an amazing destination to visit, but the system also provides drinking water for about 40% of the US population.

The caves’ first explorers were Native Americans, who mined (采矿) the upper levels of Mammoth Cave. Later, the sites were left in great shape and began to draw public interest.

Mammoth Cave was made into a national park thanks to strong support. Back in the 1920s, the Southern Appalachian National Park Commission was set up in 1925. After years of work, as well as building roads and public facilities, the park was completed on July 1, 1941.

The park now receives around 2 million visitors a year, with about a quarter taking a tour of the caves—everything from a cultural tour to the more adventurous tours on offer. The cave supports more than 130 wildlife species (物种). The park is also much more than just a cave; the forest around has a diverse (多种多样的) range of species. There are miles of trails open for horseback riding, with canoe trails and great fishing spots. There are also more than 1,300 flowering species in the forest, and animals like wood warblers, thrushes, and bald eagles can regularly be seen.

Ladies and gentlemen, welcome to the famous tourist city—

7 . Change can be unsettling. Good or bad, exciting or unexpected, there’s an undeniable anxiety that accompanies seeing our lives shift. Thankfully, nature reminds us that entering a new season in our lives is nothing to fear but something to enjoy.

What’s now known as “leaf-peeping” has become a highly profitable form of tourism in the United States. A research shows that across states in the eastern half of the country, fall foliage (叶子) tourism contributes more than $30 billion to local economies. And even though climate change has made int more difficult to predict when exactly foliage will peak, it hasn’t stopped people from trying to figure it out. Tourism leaders from the Smoky Mountains recently released their annual fall foliage prediction map which helps travelers plan out when they have the best chance to catch those beautiful autumn views.

Perhaps part of what makes the foliage so special is the belief that it’s temporary. Eventually, branches will become bare, and we’ll stop taking photos for social media. And as the seasons change and we feel that familiar longing for what was, trees will once again embrace (欣然接受) their evolution.

Even when they aren’t decorated with flashy fall colors and their internal systems slow down, trees are still supporting the ecosystem. Their fallen leaves are broken down by bacteria and fungi and provide the soil with nutrients the tree needs to grow. Insects take up shelter in their bark, eventually becoming food for other wildlife seeking a place to nest. Their strong roots stabilize the soil, preventing erosion (侵蚀) from winter snowmelt.

Right now, trees around the country are already beginning to welcome their next chapter. During this time of the year, it feels like forests are just offering endless joy for free. And fall foliage is an everlasting reminder that the growth with changes is beautiful.

Nowadays, with the increasing variety of travel options, conventional group tours,     1     going to scenic spots and taking photos is a big thing, can no longer satisfy a person     2     (look) for a more personalized and differentiated travel experience and eager to find more information about every part of a city in a more immersive (沉浸式的) way.

City-walk, which is all about doing whatever you want in your own     3     (explore) of a city, can meet all these needs, and     4     (consider) the best way to rediscover a city.

Compared with     5     (tight) scheduled travel plans and picture-taking tours, city-walk focuses on the free, casual and spontaneous (心血来潮的) experience. It allows people to see     6     whole different side of a city, permitting them to experience urban life with no transportation other than     7     (one) legs and feet.

    8     a kind of slow-paced and low-carbon lifestyle, city-walk provides people with fresh new insights into the culture and landscape of a city as well as its beautiful natural scenery, and     9     (catch) the public’s eye on various social media platforms during the past months. The search for city-walk is reported     10     (be) on the increase and more people tend to share their city-walking experiences online.