the World’s Largest Prehistoric Construction Projects

Some might suppose that one of the world's largest prehistoric construction projects is the Great Pyramid at Giza. With a height of 146 m (481 ft), the Great Pyramid is indeed massive, holding the title of world's tallest building for over 3,800 years. However, the Great Pyramid was built around 2560 BCE, at least 800 years into the historic era of Egypt, which began around 3200 BCE. By our standards, this is almost a prehistoric construction project, but not quite. The historic era is defined as one in which records were kept, and they were — we know that the pyramid was constructed for the pharaoh Khufu, and probably designed by his vizer Hemon.
Although Egypt's historic era began around 3200 BCE, many other areas of the world still lacked writing, relying on crude pictograms to relay concepts. For instance, around 2600 to 1900 BCE, in modern-day Pakistan and far West India, an ancient civilization, the Indus Valley Civilization, was flourishing. This civilization built large cities for thousands of people, which included canals, temples, public baths, sewers, granaries, dockyards, and warehouses. Considered collectively, these prehistoric construction projects were among the world's largest up to the that point. However, unlike many contemporary and later civilizations, the Indus Valley people did not build monumental structures such as temples or palaces. The largest uncovered structure is a public bath.
Another very old structure, one of the prehistoric construction projects, is Silbury Hill in England, dated to 2750 ±95 BC. The massive man-made mound is 167 m (550 feet) in diameter and 40 m (131 ft) tall. Its purpose is unknown. Archaeologists have stated that the hill took 18 million man-hours, or 500 men working 15 years to build. More likely, its construction involved several thousand working in shifts for at least a couple decades. This kind of social structure is not typically associated with Neolithic Britain, which is thought to have been composed mainly of fragmented tribes. Perhaps some elite group gained control over a wide area and used its population to complete this amazing prehistoric construction project.