The Tribute in Light is seen behind the Empire State Building on September 10, 2011 in New York City. The Tribute in Light is an art installation honoring those who perished in the 9/11 attacks.
- [Mario Tama/AFP/Getty Images]

Visitors walk out on the Ledge, a glass cube that juts out from the 103rd floor Skydeck of the Sears Tower — now the Willis Tower — in Chicago, Ill.
- [Scott Olson/AFP/Getty Images]

A picture taken 20 May 1986 in New York shows the Statue of Liberty and the Twin Towers of the World Trade Center in the background. The Twin Towers collapsed 11 September 2001 after being damaged by two hijacked planes.
- [MARIO SURIANI/AFP/Getty Images]

People admire the city from the terrace roof of the Empire State Building in Manhattan, December 1946 at New York. Designed by the firm of Shreve, Lamb & Harman, the Empire State Building was built in 1930. At 1472 feet, it was the tallest building in the world — until 1970.
- [STAFF/AFP/Getty Images]

The rubble of the World Trade Center following the September 11 attacks.
- [AFP/AFP/Getty Images]

The twin towers of the World Trade Center after hijacked airliners crashed into them on September 11, 2001.
- [HENNY RAY ABRAMS/AFP/Getty Images]

The Empire State Building is illuminated with green lights at sunset in honor of the Muslim holiday Eid-al-Fitr, marking the end of the holy month of Ramadan.
- [STAN HONDA/AFP/Getty Images]

A view of the Empire State Building at sunset.
- [Julian Finney/AFP/Getty Images]

A construction crane is parked near the Petronas towers in Kuala Lumpur. Malaysia slashed its 2009 growth forecast from 5.4 percent to 3.5 percent, and announced a 2.0 billion dollar spending program to boost the economy.
- [SAEED KHAN/AFP/Getty Images]

The Willis Tower, once known as the Sears Tower, rises above the Chicago skyline. The building, once the tallest in the world, was built in the 1970s to house the offices of the then largest retailer in the world Sears, Roebuck & Co.
- [Scott Olson/AFP/Getty Images]

Sam Kane lays on the floor of the Ledge, a glass cube that juts out from the 103rd floor Skydeck of Chicago's Willis Tower, 1,353 feet above the ground.
- [Scott Olson/AFP/Getty Images]

Malaysia's landmark Petronas Twin Towers in Kuala Lumpur at night. The Petronas Twin Towers were the world's tallest buildings before being surpassed by Taipei 101. However, the towers are still the tallest twin structures in the world.
- [SAEED KHAN/AFP/Getty Images]

The Taipei 101 building in Taipei, Taiwan.
- [PATRICK LIN/AFP/Getty Images]

The Empire State Building (L) in New York, Big Ben (C) in London and the Eiffel Tower (R) in Paris with lights turned off during "Earth Hour" on March 28, 2009, a movement to promote efforts to thwart climate change.
- [TIMOTHY A. CLARY/AFP/Getty Images]

Dubai's Burj Khalifa building, currently the world's tallest tower, in the United Arab Emirates.
- [JOEL SAGET/AFP/Getty Images]

The Burj Khalifa, the world tallest tower, silhouetted at sunset in Dubai. The construction and real-estate sectors in Dubai registered a near five-percent fall in 2010, a top Dubai government official said, adding that the sector was hit hard by the financial crisis.
- [MARWAN NAAMANI/AFP/Getty Images]

Fireworks light up the sky near the landmark Petronas Twin Towers (R) during New Year celebrations in Kuala Lumpur on January 1, 2011.
- [AFP/AFP/Getty Images]

Early morning light hits the Empire State Building on 5th Avenue in New York March 14, 2011.
- [TIMOTHY A. CLARY/AFP/Getty Images]

French climber Alain Robert scales the 828-meter Burj Khalifa in Dubai on March 28, 2011. French 'Spiderman' Robert is world famous for illegally climbing tall buildings across the world.
- [MARWAN NAAMANI/AFP/Getty Images]

French climber, 'Spiderman' Alain Robert on his way up the Burj Khalifa.
- [MARWAN NAAMANI/AFP/Getty Images]

The Empire State Building is illuminated in white and red, the colors of the Japanese national flag, to show solidarity with the people of earthquake and tsunami-hit Japan, on April 4, 2011 in New York City.
- [STAN HONDA/AFP/Getty Images]

A general view of the sunset over the New York City skyline is seen from the Arthur Ashe Stadium on the grounds of the USTA Billie Jean King National Tennis Center during Day Thirteen of the 2011 US Open on September 10, 2011 in the Flushing neighborhood of the Queens borough of New York City.
- [Nick Laham/AFP/Getty Images]

The North Pool of the 9/11 Memorial, with One World Trade Center under construction at rear during the tenth anniversary ceremonies of the September 11, 2001 terrorist attacks at the World Trade Center site, September 11, 2011 in New York City. New York City and the nation are commemorating the tenth anniversary of the terrorist attacks on lower Manhattan which resulted in the deaths of 2,753 people after two hijacked planes crashed into the World Trade Center.
- [Pool/AFP/Getty Images]