It depends what you consider to be a structure, or what you mean by “largest.”
The largest man-made object is the Great Pacific Garbage Patch. It's hard to pin down the exact size of the patch, but some estimates put it at around 550,000 square miles, or about twice the size of Texas. Currents across the Pacific Ocean drag trash from the coasts of Asia and North America to this area creating a soup of debris.
The largest man-made lake will soon be the reservoir created by China's Three Gorges Dam. Although the dam's structure was completed in 2006, the reservoir is still growing. This as-yet unnamed lake will be about the same size as Lake Superior, the world's largest fresh water lake by land area. 1.4 million people had to be moved out of the 350 mile stretch of land the reservoir will cover.
The largest structure measured by volume and floor area is the Boeing Everett factory. It's over 472 million cubic feet covering 4.3 million square feet of floor space. Originally a forty-two acre complex built for the manufacture of the 747, the plant expanded to accommodate production of the the 767, 777, and 787 Dreamliner. It now covers just under one hundred acres of land.
The heaviest man-made object is the Great Pyramid of Cheops. The oldest of the Great Pyramids of Giza, its 2.3 million stone blocks weigh an estimated six million tons.
The longest man-made object is the South-East Asia - Middle East - Western Europe 3 telecommunications cable at 24,000 miles. The SEA-ME-WE 3 starts on the north coast of Germany, wraps around Europe to pass under the Mediterranean Sea, through the Red Sea, and along the southern coast of Asia with branches running south to Australia and north to Japan. Despite its length, the entire span of cable weighs just over one third as much as the Great Pyramid of Cheops.
The largest moving man-made object was the Mont. Also named the Seawise Giant, Happy Giant, Jahre Viking, and Knock Nevis during its service, the supertanker was 1,504 feet long, nearly twice as long as the Titanic. When fully loaded she weighed almost 650,000 tons. Top Gear host Jeremy Clarkson visited the ship in 1997 for his TV show "Extreme Machines."
The ship was scrapped in 2010 but her thirty-six ton anchor was preserved. It is now on display at the Hong Kong Maritime Museum.
Posted 3255 day ago