On the western side of Mount Hood lies the longest glacier cave system in the contiguous United States. In 2012, these caves were mapped to a combined length of 7,166.8 feet by cave explorers Brent McGregor and Eddy Cartaya. Currently, the total passage length is hundreds of feet less. Glaciers are frozen rivers; they are always moving and changing... (Photo and caption by Josh Hydeman)

A hyena eyes a herd of zebra at Lake Nakuru National Park, Kenya, August 19, 2015. The Park is home to some of the world's most majestic wildlife including lions, rhinos, zebras and flamingos. The scenery is stunning, from forests of acacia trees to animals congregating at the shores to drink. UNESCO says that with rapid population growth nearby, the area is under "considerable threat from surrounding pressures," particularly deforestation, a contributing factor in floods. (Photo by Joe Penney/Reuters)

Actors perform the living Nativity biblical scenes in Postojna cave in Postojna, Slovenia, December 22, 2015. (Photo by Srdjan Zivulovic/Reuters)

Located in Siberia, Laka Baikal is the largest freshwater lake on the planet that contains approximately 20 percent of the Earth’s freshwater. Apart from being the oldest lake in the world at over 25 million years old, Lake Baikal is also home to over two thousand varieties of flora and fauna, of which almost 1,600 are endemic to the lake. The water of Lake Baikal is renowned for being some of the clearest in the world. When the lake freezes during the winter, an amazing phenomena takes place: large shards of transparent ice form on the surface of the lake, giving the amazing appearance of turquoise ice.

Ellora is an archaeological site, 29 km (18 mi) North-West of the city of Aurangabad in the Indian state of Maharashtra built by the Rashtrakuta dynasty. It is also known as Elapura (in the Rashtrakuta literature-Kannada).

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Lake Natron is a salt lake located in northern Tanzania, close to the Kenyan border, in the eastern branch of the East African Rift. The lake is fed by the Southern Ewaso Ng'iro River and also by mineral-rich hot springs. It is quite shallow, less than three meters (10 feet) deep, and varies in width depending on its water level, which changes due to high levels of evaporation, leaving concentrations of salt and other minerals, notably sodium carbonate (natron).

Yueyaquan is a crescent-shaped lake in an oasis, 6 km south of the city of Dunhuang in Gansu Province, China. It was named Yueyaquan in the Qing Dynasty. According to measurements made in 1960, the average depth of the lake was 4 to 5 meters, with a maximum depth of 7.5 metres In the following 40 years, the depth of the lake continually declined. In the early 1990s, its area had shrunk to only 5,500 m2 with an average depth of 0.9 meter (maximum 1.3 meter). In 2006, the local government with help of the central government started to fill the lake and restore its depth; its depth and size have been growing yearly since then. The lake and the surrounding deserts are very popular with tourists, who are offered camel and 4x4 rides.

Located in the Mokattam Mountain in southeastern Cairo, Egypt, the Monastery of Saint Simon or the Cave Church is a beautiful and mystical location. The area is also known under the name of the “garbage city” because of the large population of garbage collectors or Zabbaleen that live here and who are the descendants of farmers who started migrating from Upper Egypt to Cairo in the 1940s.