Great Wall of China: Beijing, China

The Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China to protect the Chinese states and empires against the raids and invasions of the various nomadic groups of the Eurasian Steppe. Several walls were being built as early as the 7th century these, later joined together and made bigger and stronger, are now collectively referred to as the Great Wall. Especially famous is the wall built 220–206 bce by Qin Shi Huang, the first Emperor of China. Little of that wall remains. Since then, the Great Wall has on and off been rebuilt, maintained, and enhanced; the majority of the existing wall is from the Ming Dynasty.

Other purposes of the Great Wall have included border controls, allowing the imposition of duties on goods transported along the Silk Road, regulation or encouragement of trade and the control of immigration and emigration. Furthermore, the defensive characteristics of the Great Wall were enhanced by the construction of watch towers, troop barracks, garrison stations, signaling capabilities through the means of smoke or fire, and the fact that the path of the Great Wall also served as a transportation corridor.

The Great Wall stretches from Dandong in the east, to Lop Lake in the west, along an arc that roughly delineates the southern edge of Inner Mongolia. A comprehensive archaeological survey, using advanced technologies, has concluded that the Ming walls measure 8,850 km (5,500 mi).This is made up of 6,259 km (3,889 mi) sections of actual wall, 359 km (223 mi) of trenches and 2,232 km (1,387 mi) of natural defensive barriers such as hills and rivers.Another archaeological survey found that the entire wall with all of its branches measure out to be 21,196 km (13,171 mi)

The Arctic Circle

The Arctic Circle is the most northerly of the five major circles of latitude that mark maps of the Earth. The region north of this circle is known as the Arctic, and the zone just to the south is called the Northern Temperate Zone. North of the Arctic Circle, the sun is above the horizon for twenty-four continuous hours at least once per year (and therefore visible at midnight) and below the horizon for twenty-four continuous hours at least once per year (and therefore not visible at noon); this is also true within the equivalent polar circle in the Southern Hemisphere, the Antarctic Circle.

The position of the Arctic Circle is not fixed; as of 28 June 2016, it runs 66°33′46.3″ north of the Equator.Its latitude depends on the Earth's axial tilt, which fluctuates within a margin of 2° over a 40,000-year period, due to tidal forces resulting from the orbit of the Moon.Consequently, the Arctic Circle is currently drifting northwards at a speed of about 15 m (49 ft) per year.

Grand Canyon National Park: Arizona, USA

Grand Canyon National Park is the United States' 15th oldest national park. Named a UNESCO World Heritage Site in 1979, the park is located in northwestern Arizona. The park's central feature is the Grand Canyon, a gorge of the Colorado River, which is often considered one of the Seven Natural Wonders of the World. The park covers 1,217,262 acres (1,901.972 sq mi; 492,608 ha; 4,926.08 km²) of unincorporated area in Coconino and Mohave counties.

History

Grand Canyon was officially designated a national park in 1919, though the landmark had been well known to Americans for over thirty years prior. In 1903, President Theodore Roosevelt visited the site and said: "The Grand Canyon fills me with awe. It is beyond comparison—beyond description; absolutely unparalleled through-out the wide world... Let this great wonder of nature remain as it now is. Do nothing to mar its grandeur, sublimity and loveliness. You cannot improve on it. But what you can do is to keep it for your children, your children's children, and all who come after you, as the one great sight which every American should see."

Despite Roosevelt's enthusiasm and his strong interest in preserving land for public use, the Grand Canyon was not immediately designated as a national park. The first bill to establish Grand Canyon National Park was introduced in 1882 by then-SenatorBenjamin Harrison, which would have established Grand Canyon as the second national park in the United States afterYellowstone. Harrison unsuccessfully reintroduced his bill in 1883 and 1886; after his election to the presidency, he established the Grand Canyon Forest Reserve in 1893. Theodore Roosevelt created the Grand Canyon Game Preserve by proclamation on 28 November 1906 and Grand Canyon National Monument in 1908. Further Senate bills to establish the site as a national park were introduced and defeated in 1910 and 1911, before the Grand Canyon National Park Act was finally signed by PresidentWoodrow Wilson in 1919. The National Park Service, established in 1916, assumed administration of the park.

The creation of the park was an early success of the conservation movement. Its national park status may have helped thwart proposals to dam the Colorado River within its boundaries. (Later, the Glen Canyon Dam would be built upriver.) In 1975, the former Marble Canyon National Monument, which followed the Colorado River northeast from the Grand Canyon to Lee's Ferry, was made part of Grand Canyon National Park. In 1979, UNESCO declared the park a World Heritage Site.

In 2010, Grand Canyon National Park was honored with its own coin under the America the Beautiful Quarters program.

Serengeti National Park: Tanzania

The Serengeti National Park is a Tanzanian national park in the Serengeti ecosystem in the Mara and Simiyu regions.It is famous for its annual migration of over 1.5 million white bearded (or brindled) wildebeest and 250,000 zebra and for its numerous Nile crocodile and honey badger.

History

The Maasai people had been grazing their livestock in the open plains of eastern Mara Region, which they named "endless plains", for around 200 years when the first European explorer, Austrian Oscar Baumann, visited the area in 1892. The name "Serengeti" is an approximation of the word used by the Maasai to describe the area, siringet, which means "the place where the land runs on forever".

The first Briton to enter the Serengeti, Stewart Edward White, recorded his explorations in the northern Serengeti in 1913. He returned to the Serengeti in the 1920s and camped in the area around Seronera for three months. During this time, he and his companions shot 50 lions.

Because the hunting of lions made them scarce, the British colonial administration made a partial game reserve of 800 acres (3.2 km²) in the area in 1921 and a full one in 1929. These actions were the basis for Serengeti National Park, which was established in 1951.

The Serengeti gained more fame after the initial work of Bernhard Grzimek and his son Michael in the 1950s. Together, they produced the book and film Serengeti Shall Not Die, widely recognized as one of the most important early pieces of nature conservation documentary.

To preserve wildlife, the British evicted the resident Maasai from the park in 1959 and moved them to the Ngorongoro Conservation Area. There is still considerable controversy surrounding this move, with claims made of coercion and deceit on the part of the colonial authorities.

The park is Tanzania's oldest national park and remains the flagship of the country's tourism industry, providing a major draw to the Northern Safari Circuit encompassing Lake Manyara National Park, Tarangire National Park, Arusha National Park and theNgorongoro Conservation Area. It has over 2,500 lions and more than 1 million wildebeest.

Grand Prismatic Spring: Yellowstone National Park, Wyoming

The Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the United States, and the third largest in the world,after Frying Pan Lake in New Zealand and Boiling Lake in Dominica. It is located in the Midway Geyser Basin.

Grand Prismatic Spring was noted by geologists working in the Hayden Geological Survey of 1871, and named by them for its striking coloration. Its colors match the rainbow dispersion of white light by an optical prism: red, orange, yellow, green, and blue.

History

The first records of the spring are from early European explorers and surveyors. In 1839, a group of fur trappers from theAmerican Fur Company crossed the Midway Geyser Basin and made note of a "boiling lake", most likely the Grand Prismatic Spring,^[5] with a diameter of 300 feet (90 m). In 1870 the Washburn–Langford–Doane Expedition visited the spring, noting a 50-foot (15 m) geyser nearby (later named Excelsior).

Color

The vivid colors in the spring are the result of pigmented archaea in the microbial mats that grow around the edges of the mineral-rich water. The archaea produce colors ranging from green to red; the amount of color in the microbial mats depends on the ratio of chlorophyll to carotenoids and on the temperature of the water which favors one archaea over another. In the summer, the mats tend to be orange and red, whereas in the winter the mats are usually dark green.The center of the pool is sterile due to extreme heat.

The deep blue color of the water in the center of the pool results from the scattering of blue light by particles suspended in the water.This effect is particularly visible in the center of the spring due to the lack of archaea that live in the center and the depth of the water.

Physical structure

The spring is approximately 370 feet (110 m) in diameter and is 121 feet (40 m) deep. The spring discharges an estimated 560 US gallons (2,100 L) of 160 °F (70 °C) water per minute.