More than 4,000 years ago builders carved out the entire surface of a naturally pyramid-shaped promontory on the Greek island of Keros. They shaped it into terraces covered with 1,000 tonnes of specially imported gleaming white stone to give it the appearance of a giant stepped pyramid rising from the Aegean: the most imposing manmade structure in all the Cyclades archipelago.

But beneath the surface of the terraces lay undiscovered feats of engineering and craftsmanship to rival the structure’s impressive exterior. Archaeologists from three different countries involved in an ongoing excavation have found evidence of a complex of drainage tunnels – constructed 1,000 years before the famous indoor plumbing of the Minoan palace of Knossos on Crete – and traces of sophisticated metalworking.

The Dhaskalio promontory is a tiny island as the result of rising sea levels, but 4,500 years ago was attached by a narrow causeway to Keros, now uninhabited and a protected site. In the third millennium BC Keros was a major sanctuary where complex rituals were enacted. Earlier excavations by the team from the University of Cambridge, the Ephorate of Antiquities of Cyclades and the Cyprus Institute have uncovered thousands of marble Cycladic sculptures – the stylised human figures which inspired western artists, including Pablo Picasso – and which appear to have been deliberately broken elsewhere and brought to the island for burial.