Construction work on the tower began in November 2008. Upon its completion this year the building will stand approximately 632 meters (2,073 ft.) high and will have 121 stories. It is expected to open to the public in 2015. (Photo by Rex Features)

Twin baby panthers Remaong and Ferra cling to their basket while being presented to the media at Tierpark Berlin zoo on July 13, 2012 in Berlin, Germany. The two panthers were born in captivity on April 26. (Photo by Sean Gallup)

A polar bear plays with a pylon during celebrations marking its first birthday in an enclosure at Tierpark Hellabrunn zoo in Munich December 9, 2014. (Photo by Michaela Rehle/Reuters)

A street in the Cuban capital, Havana, is home to 12 sets of twins, ranging from toddlers to senior citizens. Some say it could be something in the water. Others point to a tree with mystical significance for locals. And maybe it's just chance. Photo: In this September 29, 2013 photo, nine-year-old twin sisters Camila, left, and Carla Rodriguez pose for a portrait along their street in Havana, Cuba. 12 sets of twins live along two consecutive blocks in western Havana, ranging in age from newborns to senior citizens. “We love living on this block because we have twin friends”, said Carla. (Photo by Ramon Espinosa/AP Photo)

Conjoined twins Angelica (C) and Angelina Sabuco (L) play with child life specialist Allison Brooks during a press conference at the Lucile Packard Children's Hospital at Stanford on October 31, 2011 in Palo Alto, California. Two year-old conjoined twins Angelica and Angelina Subaco who are connected at the chest and abdomen are preparing for separation surgery by Dr. Gary Hartman. The surgery will last between 6 and 8 hours that is followed by three hours of reconstructive surgery for each girl. (Photo by Justin Sullivan/Getty Images)

Maintenance engineer Darren Unsworth cleans one of the crystal Edwardian chandeliers that illuminate Blackpool Tower Ballroom on January 16, 2012 in Blackpool, England. Blackpool's famous Tower Ballroom is undergoing an annual spring clean which includes lowering the ornate chandeliers where each shard of crystal is cleaned for the coming season of dancing. The dance floor consisting of 30,602 blocks of mahogany, oak and walnut is also sanded and polished to give the perfect dance floor. (Photo by Christopher Furlong/Getty Images)

Construction continues on One World Trade Center (TALLEST BUILDING AT LOWER LEFT) as the memorial footprints of the twin towers are seen (BOTTOM C) on August 12, 2011 in New York City. Upon completion, One World Trade Center will be New York's tallest skyscraper, topping out at a symbolic 1,776 feet, with 3 million square feet of office space. More than 2,700 people were killed when al-Qaeda terrorists hijacked U.S. passenger jets and flew them into the twin towers of the World Trade Center on September 11, 2001. Nearly ten years after the crippling attacks on Lower Manhattan, business, tourism and new construction like One World Trade Center have rejuvenated the formerly devastated cityscape.(Photo by Mario Tama/Getty Images)