10 most difficult North America 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.

There were the Olympic Games in St. Louis (1904), twice in Los Angeles (1932, 1984) and in Atlanta (1996)

Martin Van Buren was the eighth president of the United States. 'Van' suggests Dutch origin of his surname, but the capitalized form indicates anglicised version - like Eddie Van Halen (as opposite to Dutch lower-case Vincent van Gogh).

White Sands, having 442 total square kilometers of dune fields, is the world's largest surface deposit of gypsum. Unlike other desert sands, it is cool to the touch, due to the high rate of evaporation of surface moisture and the fact that the sands reflect, rather than absorb, the sun's rays.

Yukon, having 1,979 mil (3,185 km), is just 223 miles shorter than Mississippi. Despite its length, there are only four vehicles-carrying bridges across the river.

Costa Rica is the only country without a military, and has banned a military since 1949. Unlike other small countries, it has no defence agreement with another country, and is defended by the police.

The Mackenzie River is the longest river system in Canada, and has the second largest drainage basin of any North American river after the Mississippi River. The river's main stem is flowing north-northwest from Great Slave Lake into the Beaufort Sea, a marginal sea of the Arctic Ocean, where it forms a large delta at its mouth.

The colors come from the national flag. 24 stars commemorate Missouri was 24th state of USA. The bears symbolized size and power of Missouri. The coat of arms means membership to Union, and crescent is symbol of new state on Lousiana territory. Adapted on 23rd March 1913

In a three year journey between 1903 and 1906, Amundsen explored the passage with a crew of no more than six, in the converted 47-ton herring boat Gjøa. Spending winters with ship trapped in ice, he learned from the local Netsilik people about Arctic survival skills that would later prove useful. For example, he learned to use sled dogs and to wear animal skins in lieu of heavy, woolen parkas. Amundsen later led the expeditions to discover the South Pole (1911) and the North Pole (1926).

The gradient between The Great Lakes is generally very shallow. However, the descent between Erie and Ontario is greater than between any of the others, and includes the spectacular Niagara Falls, 51m high.