10 most difficult tourist at­trac­tions riddles

The city is ancient - its largest pyramid, the Pyramid of the Sun, was completed by AD 100. It predates most known cultures of Mesoamerica. The origin of its founders is uncertain, and its original name is also unknown. It housed a population of about 150,000 people, making it one of the largest cities in the world at that time.

Washington Union Station is a major train station, transportation hub, and leisure destination in Washington, D.C. Opened in 1907, it is Amtrak's headquarters and the railroad's second-busiest station with annual ridership of just under 5 million and the ninth-busiest in overall passengers served in the United States. Daniel Burnham, a famed Chicago architect wrote; "The station and its surroundings should be treated in a monumental manner, as they will become the vestibule of the city of Washington, and as they will be in close proximity to the Capitol itself."

La Tomatina is a festival that is held in the Valencian town of Buñol, a town located in the East of Spain 30 kilometres (19 mi) from the Mediterranean, in which participants throw tomatoes and get involved in this tomato fight purely for entertainment purposes. Since 1945 it has been held on the last Wednesday of August, during the week of festivities of Buñol.

Đavolja varoš (Devil's Town) is a peculiar rock formation, located in south Serbia on the Radan Mountain. It features 202 exotic formations described as earth pyramids. These formations were created by strong erosion of the soil that was scene of intense volcanic activity millions of years ago. Most of the towers have "caps" or "heads" of andesite, which protect them from further erosion.

Prambanan is dedicated to the Trimurti, the expression of God as the Creator (Brahma), the Preserver (Vishnu) and the Transformer (Shiva). It is the largest Hindu temple site in Indonesia, which attracts many visitors from around the world. It is a UNESCO World Heritage Site since 1991.

The Eternal Flame Falls is located in the section of Chestnut Ridge Park in Western New York. A small grotto at the waterfall's base emits natural gas, which can be lit to produce a small flame. This flame is visible nearly year round, although it can be extinguished and must occasionally be re-lit.

The Jellyfish Lake is a marine lake located on the Eil Malk island in Palau. It is filled with millions of golden jellyfish, migrating across the lake every day following the arc of the sun. The lake is stratified into two layers, an oxygenated upper layer and a lower anoxic layer, hence scuba diving is forbidden so the two wouldn't mix. But snorkelling is allowed and a very popular activity for tourists in Palau.

In the years of splendor in Venice there was a law that allowed only one building to wear the name of the palace - it was the Doge's Palace, the residence of the rulers of Venice. The rich Venetian families lived according to this law in the so-called houses (Casa abbreviated Ca). Currently, most residences of Venetian patricians carry the word "palace" in their name, although there are exceptions and some Venetian monuments are still called according to old traditions, eg Ca'Rezzonico or Ca 'd'Oro. Over 100 survived to modern times.

Hang Sơn Đoòng is a solutional cave in Phong Nha-Kẻ Bàng National Park, Vietnam near the Laos-Vietnam border. Its total length is approx. 9 km and its main passage is the largest known in the world, being more than 5 km long, 200 m high and 150 m wide. It was discovered in 1991 and the first tourist group explored the cave in 2013.

Las Lajas Sanctuary is a basilica church located in the southern Colombian, and built inside the canyon of the Guáitara River. The present church was built in Gothic Revival style between 1916 and 1949. The name Laja comes from the name of a type of flat sedimentary rock similar to shale and slate.It rises 100 metres high from the bottom of the canyon and is connected to the opposite side of the canyon by a 50 metres tall bridge.