Libya's £14 billion Great Man-made River is one of the world's largest water-supply projects. It involves mining ancient groundwater reservoirs under the Sahara desert and piping it to the coast, where most of the country's 6 million inhabitants live. The first and largest two phases have been operating for over 10 years, two more sections are under construction and future phases will bring total deliveries to 6·6 million m3 a day. This paper provides an overview of the project to date and of the many lessons learned from its construction and operation. It also compares original environmental impact projections with impacts actually experienced—which indicate reservoir depletion is proving slower than expected such that supplies should last well into the next century. Libya's £14 billion Great Man-made River is one of the world's largest water-supply projects. It involves mining ancient groundwater reservoirs under the Sahara desert and piping it to the coast, where most of the country's 6 million inhabitants live. The first and largest two phases have been operating for over 10 years, two more sections are under construction and future phases will bring total deliveries to 6·6 million m3 a day. This paper provides an overview of the project to date and of the many lessons learned from its construction and operation. It also compares original environmental impact projections with impacts actually experienced—which indicate reservoir depletion is proving slower than expected such that supplies should last well into the next century.