Taj Mahal, Agra, India As many as 28 different varieties of semi-precious and precious stones were used to decorate the outside of the Taj Mahal. Construction of the Taj Mahal took around 20 years. The building, which was made from white marble from the quarries  of Rajasthan, appears pink in the morning, white in the day and golden in the moonlight. Akashi Kaikyo Bridge, Japan It took 10 years to construct the Akashi Kaikyo Bridge. It connects the city of Kobe, on Japan's mainland, with Iwaya on Awaji Island. Before it opened, the only way to get between the two cities was ferry. However, the waterway was prone to   severe storms and when two ferries capsized   in 1955, killing 168 people, public anger convinced   the government of the need for a bridge. It's the longest suspension   bridge in the world, with a length of 1,991 meters. Panama Canal, Panama More than 4.5 million cubic yards of concrete were used in the construction of this canal's locks and dams. The Panama Canal is a 77-kilometer (47-mile) long waterway that connects the Atlantic and Pacific Oceans. The earth and rubble excavated   to make way for it was enough to bury Manhattan to a depth of four meters. A series of locks allow ships to pass through the water; each of the moveable lock doors weighs 750 tons and each of the locks fills with 52 million gallons of water to accommodate the 15,000 ships that use the canal every year. Completion date: January 7, 1914 after 10 years of building. Grand Canyon Skywalk, Arizona, America The Skywalk's foundation is strong enough to support 71 million pounds. Located 1, 219 meters above the Colorado River, the Grand Canyon's Skywalk consists of one million pounds of steel and 83,000 pounds of glass. It was the creation of Las Vegas businessman David Jin, who approached the Hualapai Tribe with the idea of a glass walkway over the Grand Canyon in 1996. The Skywalk was assembled   on site. Completion date: March 28, 2007. Which construction took the longest time to build?
The Taj Mahal. The Panama Canal. The Akashi Kaikyo Bridge. The Grand Canyon Skywalk.