World's wonders, worn down? Only one of the original Seven Wonders still exists. Find out what threatens today's most precious monuments

How do you decide what places to visit when you go on vacation? The ancient Greeks made a list. This ancient travel guide contained the must-see statues, monuments, and places near Greece. These sites came to be known as the Seven Wonders of the World. Since then, wars, accidents, fires, and earthquakes have taken their toll, leaving only one ancient wonder--the Pyramids of Giza--still standing.

A group called the New7Wonders Foundation thinks it's time to update the top-seven list, and they want everyone's opinion to count. They have pared the list to 21 present-day wonders front around the globe (see map, p. 15). Would-be voters can go online to choose which historical sites should make the final cut.

How are the top contenders holding up to the test of time? Will they stiffer fates similar to the original Seven Wonders? Science World spoke with experts to find out how some of the sites are faring, and what's being done to protect them.

1 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, 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.

2 PHARAOHS' TOMBS

Pyramid of Giza--Egypt

PROBLEM: WEATHERING

These colossal 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.

3 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 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. 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 are working to dig out the buildings. "Despite being in an earthquake-prone area, most of the buildings carved out of rock are still standing," she says.

4 MONUMENT TO LOVE

Taj Mahal--India

4 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 formed when heat and pressure underground cause minerals to crystallize. 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 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.

5 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 the tower every seven years.