Answer: Libya

The Romans might have built aqueducts that are still standing today and modern agriculture the world over might depend on advanced irrigation projects, but there’s no ancient or modern project that can even hold a candle to Libya’s staggering-in-scope Great Man-Made River project.

In the 1950s while conducting exploratory drilling for oil reserves, the Libyan government found, instead of oil, a vast underground aquifer. This aquifer, the Nubian Sandstone Aquifer System, is the largest fossil water aquifer system in the world with a footprint of over 772,000 million square miles and an estimated 36,000 cubic miles of groundwater.

After decades of planning, the Great Man-Made River project started in 1984, funded by the Gaddafi-run government. The work has been divided into five phases; the first of which was started in 1991 and the second of which was begun in 1996. As of now three of the five phases are complete and, even in a stage of partial completion, it is the largest irrigation project in the world with over 1,300 wells, 1,750 miles of underground pipeline and aqueducts, and it supplies 6.5 million cubic meters of fresh water per day to major cities like Tripoli, Benghazi, and Sirte.

Despite the scope of the project and the volume of water it moves, the Nubian Sandstone Aquifer System is so vast that it will take hundreds of years to deplete it.