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 preventederosion by stopping soil from washing down the mountainsides. But a different typeof erosion now threatens the city: The pounding feet of tourists are damaging the city'sfragile stones. Thousands visit the city every day, says MichelleBerenfeld, a program Manager for the World Monuments Fund.

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 manyimmigrants saw when they arrived in America and [is] a symbol of freedom," says Pamela Jerome, a professor of historic preservation atColumbia University in New York. The thin copper layer covering the statue was once thecolor of a penny. This layer is now blue-green—the result of the metal reacts with water and air to create copper carbonate. This naturallyoccurring coating protects the remaining copperunderneath. Water also corroded much of thestatue's original iron skeleton. To ensure LadyLiberty's safety, beginning in 1984, iron pieceswere replaced with the alloy stainless steel--a durable mixture of iron, carbon, and other metals.

Pyramid of Giza-Egypt PROBLEM: WEATHERING

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

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, carved many of the buildings right out of the sandstone walls. But Petra's location has a major downfall; it lies in the Great Rift Valley, a geological fault system of SW Asia and E Africa. As the ground moves along this fault, it generates earthquakes. Result: More than 90 percent of the city is now buried in rubble. Despite being in an earthquake-prone area, most of the buildings carved out of rock are still standing.

MYSTERIOUS MOAI 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 rocks. 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 growingin cracks in the moist stone alsobreak down the statues.

Taj Mahal--India PROBLEM: POLLUTION

When Shah Jehan's wife died, the Muslim emperor spared no expense in building an enormous domed mausoleum fit for his queen. The Taj Mahal's gleaming, white surface is made from marble, a metamorphic rock. 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 pollutioninto the air. The pollutants have begun to eat away and discolor thewhite marble. So far, some of thedamaged sections of stone have been replaced and the Indian government is working on ways toimprove the city’s air quality.

Eiffel Tower--France PROBLEM: CORROSION

The tower is the most visited monument in the world. It was Eiffel built in 1889 to celebrate the 100th anniversaryof the French Revolution. It is the tallest structure in the Paris standing 1,063 feet tall. The tower is made entirely ofthe metal iron. Since metals are good conductors of heat,when temperatures warm up during the day the tower's metalframe expands, causing it to lean as much as 7.1 inches. Luckily, the tower was designed to bend easily. A bigger risk: Oxygen from the air and rainwater can cause iron to rust. To prevent this type of corrosion,

workers repaint the tower every seven years.

The statue is the largest art deco statue in the world. It stands 130 feet tall, with a 31 foot pedestal and is 98 feet wide. Its weight is 635 tons and is made of reinforced construction. It is located at the very top of the 2300 foot Corcovado Mountain and overlooks the city of Rio de Janeiro (Brazil). It is made of reinforced concrete and soapstone. In 2008 the statue was struck by lightening and in 2010 vandals sprayed graffiti on the head and right arm. A layer of fungi and

other microorganisms were removed and small cracks caused by ice wedging were also repaired in 2010. Routinemaintenance is required due to the strong winds and erosion.

It is visited by thousands oftourists each year.

Christ the Redeemer(One of the new 7 Wonders of the World)

PROBLEM: EROSION

Leaning Tower of PisaProblem: Built on clay - leans

Begun in 1173, the tower was built with limestone and the exterior of the tower is covered in marble. Ironically, the limestone is probably why the tower has not cracked and broken- the rock is flexible enough that it can withstand the pressures placed on it by the lean. Originally, the leaning tower of Pisa was to be a bell tower for a cathedral. The tower began to lean once the third floor was added. In 1934, 361 holes were drilled into the foundation of the tower and 90 tons of cement were used to fill them. The cement, rather than filling in the holes and acting as a counterweight, sank into the clay beneath the structure, causing the tower to lean over even more.

The Great Wall of China Problem: Erosion

The wall was originally made of earth, wood, and stone. It was built to protect China from northern invasions. Several walls were built as early as the 7th century BC and by 221 BC, the wall reached 3,107 miles. The Great Wall has been rebuilt with stones, bricks, and granite blocks. Over the past 100 years, the wall has experienced weathering due to gravity, wind, and water erosion, as well as human activity. The Great Wall of China is the only man made object that can be seen from outer space with the naked eye.

Great Wall of China

The Great Wall of China as seen from space.