The most difficult famous places of North America riddles

The cave near Naica, Mexico, was discovered in 2000 by miners. At that time, it was completely submerged. Today, the cave is accessible because the mining company's pumping operations keep it clear of water. Unfortunately, the crystals deteriorate in air, and soon may be lost.

The Gateway Arch is a 630-foot (192 m) monument in St. Louis, Missouri. It was built as a monument to the westward expansion of the United States, it is the centerpiece of the Jefferson National Expansion Memorial and has become an internationally famous symbol of St. Louis. It is the tallest man-made monument in the United States and the world's tallest arch.

Designed by Frank Lloyd Wright, the cylindrical building of the Guggenheim Museum is one of the 20th century's most important architectural landmarks. Its unique ramp gallery extends from just under the skylight in the ceiling in a long, continuous spiral until it reaches the ground level.