The largest man-made objects in the world are submarine communications cables. The longest stretch from San Francisco to New Zealand or San Francisco to Japan, stretching over 5,000 mi (8,000 km). These submarine cables are typically 2.6 in (6.6 cm) in diameter and weigh around 22 lb (10 kg) per meter, weighing in at a total of more than 176 million pounds (80,000 metric tons) for the entire length. This may sound like a lot, but it isn't much in comparison to other man-made objects: for instance, the Great Pyramid weighs about 8.3 billion pounds (3.8 million metric tons), and the Three Gorges Dam in China weighs about 75 billion pounds (34 million metric tons).
Discounting other two-dimensional objects such as rail lines, power lines, oil pipelines, and so on, some of the world's largest man-made objects are scientific experiments. For instance, the Large Hadron Collider (LHC) is a circular tunnel 16.5 mi (26.6 km) in length, buried between 164 to 574 ft (50 to 175 meters) underground, and includes over 1,600 superconducting magnets. It collides subatomic particles at to within a fraction of the velocity of light, and cost between $5 and $10 billion US Dollars (USD) to build.
Another large man-made object in the form of a scientific experiment is the IceCube neutrino detector, located at the South Pole. Consisting of a series of optical sensors deployed on strings over 0.62 miles (1 km) long, the total size of the experiment is about 0.239 cubic miles (1 cubic km).
More famously, the largest man-made object in terms of weight and 3-D volume is the Three Gorges Dam, which spans the Yangtze River in China. As previously mentioned, the dam weighs about 75 billion pounds (34 million metric tons), has a length of 7,661 ft (2,335 m) a height of 607 ft (185 m), and width (at the base) of 377.3 ft (115 m). A close runner-up is the Terminal 3 building at the Beijing Capital International Airport, which covers 240 acres (0.97 square km) and is nearly 2 miles (3.2 km) long.