CITY IN THE CLOUDS Machu Picchu--Peru

PROBLEM: EROSION

The ancient city of Machu Picchu sits atop a peak in Peru's Andes Mountains. To create space to grow food, the Incas carved terraces into the steep slopes. These steps prevented erosion by stopping soil from washing down the mountainsides. But a different type of erosion now threatens the city: The pounding feet of tourists are damaging the city's fragile stones. Thousands visit the city every day, says Michelle Berenfeld, a program manager for the World Monuments Fund. Among other things, the WMF develops plans to help tourists better navigate historical places. "Sites like Machu Picchu are the only things left to tell us about cultures that are now gone. But the more people know and appreciate a site, the more they will want to preserve it," says Berenfeld.

SYMBOL OF FREEDOM Statue of Liberty--United States

PROBLEM: CORROSION Given as a gift from France to the U.S. in 1886, the Statue of Liberty

, stands on an island in New York Harbor. "It's the first thing many immigrants saw when they arrived in America and [is] a symbol of freedom," says Pamela Jerome, a professor of historic preservation at Columbia University in New York. The thin copper layer covering the statue was once the color of a penny. This layer is now blue-green--the result of the metal reacting with water mid air to create copper carbonate. This naturally occurring coating protects the remaining copper underneath. Water also corroded much of the statue's original iron skeleton. To ensure Lady Liberty's safety, beginning in 1984, iron pieces were replaced with the alloy stainless steel--a durable mixture of iron, carbon, and other metals.

ENGINEERING MARVEL Eiffel Tower--France

PROBLEM: CORROSION

Named after its designer, Alexandre-Gustave Eiffel, this tower is the most visited monument in the world. Eiffel built the tower in 1889 to celebrate the 100th anniversary of the French Revolution. It is the tallest structure in the Paris skyline, standing 324 meters (1,063 feet) tall. The tower is made entirely of the metal iron. Since metals are good conductors of heat, when temperatures warm up during the day the tower's metal frame expands, causing it to lean as much as 28 centimeters (7.1 inches). Luckily, the tower was designed to bend easily. A bigger risk: Oxygen from the air and rainwater can cause iron to rest. To prevent this type of corrosion, high-climbing workers repaint    v. t. 1. To paint anew or again; as, to  a house; to  the ground of a picture. s>Verb 1.  the tower every seven years.

MONUMENT TO LOVE Taj Mahal--India

PROBLEM: POLLUTION When Shah Jehan's wife died, the Muslim emperor spared no expense in building an enormous domed mausoleum (môsəlē`əm), a sepulchral structure or tomb, especially one of some size and architectural pretension, so called from the sepulcher of that name at Halicarnassus, Asia Minor,

erected (c.352 B.C. .....  fit for his queen. The Taj Mahal's gleaming, white surface is made from marble, a metamorphic rock formed when heat and pressure underground cause minerals to crystallize  also crys·tal·ize  v. crys·tal·lized also crys·tal·ized, crys·tal·liz·ing also crys·tal·iz·ing, crys·tal·liz·es also crys·tal·iz·esv.tr.1. . Polishing these crystals produces a translucent, shining surface. Despite the Taj's romantic history, the modern world may be taking its toll on the monument. Factories, oil refineries, and cars in the nearby city of Agra pump pollution into the air. The pollutants have begun to eat away and discolor   v. dis·col·ored, dis·col·or·ing, dis·col·orsv.tr.To alter or spoil the color of; stain.v.intr.To become altered or spoiled in color.  the white marble. So far, some of the damaged sections of stone have been replaced and the Indian government is working on ways to improve the city's air quality.

LOST CITY Petra--Jordan

PROBLEM: EARTHQUAKES

The city of Petra is hidden deep within a desert canyon. To enter, visitors first pass through a narrow crack of rock with walls hundreds of feet high on either side. The Nabataeans, the ancient people who built the city, chiseled  or chis·elled  adj.Made or shaped with or as if with a chisel: Adj. 1.  many of the buildings right out of the sandstone walls. But Petra's location had a major downfall; it lies in the Great Rift Valley  geological fault system of SW Asia and E Africa. It extends c.3,000 mi (4,830 km) from N Syria to central Mozambique. The northernmost extension runs S through Syria and Lebanon, the Jordan valley, the Dead Sea, and the Gulf of Aqaba. . As the ground moves along this fault, it generates earthquakes. Result: More than 90 percent of the city is now buried in rubble. Archaeologists like Martha Sharp Joukowsky of Brown University in Rhode Island Rhode Island, island, United States island, 15 mi (24 km) long and 5 mi (8 km) wide, S R.I., at the entrance to Narragansett Bay. It is the largest island in the state, with steep cliffs and excellent beaches.  are working to dig out to depart; to leave, esp. hastily; decamp.See also: Dig  the buildings. "Despite being in an earthquake-prone area, most of the buildings carved out of rock are still standing," she says.

Pharaoh’s TombsPyramid of Giza--Egypt

PROBLEM: WEATHERING These colossal (massive, huge) pyramids have stood in the desert outside of Cairo, Egypt, for more than 4,000 years. Ancient Egyptians built the tombs to honor their Pharaohs--the kings whom they

believed to be gods. The blocks they used to build the pyramids were cut out of limestone, a sedimentary rock formed when layers of soil are deposited on top of each other. The pyramids once had smooth sides, but wind-blown sand has scoured away most of their outer shell. This weathering has exposed jagged inner layers. Archaeologists have learned much about ancient Egyptian culture

by examining the pyramids, though they still debate how builders put the heavy blocks in place. However the pyramids were created, they were built to last.

MYSTERIOUS MOAI MOAI Monoamine Oxidase InhibitorMOAI Ministry of Agriculture and Irrigation

MOAI Mortgages Online At Interfirst   Ester Island--Chile PROBLEM: WEATHERING

This South Pacific island is dotted with hundreds of giant statues, called Moai. Long ago, each statue was carved from blocks of the same igneous  adj.1. Of, relating to, or characteristic of fire.2. Geologya. Formed by solidification from a molten state. Used of rocks.b. Of or relating to rock so formed; pyrogenic. ..... , or volcanic, rock that produced rile island. Many of the statues are partially buried under shifting soil, so only their heads are visible. "Most people don't realize that the heads have torsos hidden underneath the ground," says Robert J. Koestler, director of the Smithsonian's Museum Conservation Institute. Being covered actually helps protect the statues, he adds. After thousands of years of rain and sea spray pelting the soft igneous stone, weathering has washed away many of the heads' facial features. Plants, bacteria, and fungi growing in cracks in the moist stone also break down the statues. One of the best-preserved Moai is on exhibit in the British Museum in London--far from Easter Island's harsh environment.