In the first century AD, the Greek geographer Strabo hypothesized that the rocky hill of Piraeus had once been an island. Located approximately seven kilometers south west of Athens, this vast rocky hill was home to the three ancient ports of the Greek capital, Zea, Mounichia and Cantharos. During the fifth century BC, this strategic place was connected to Athens by a road protected by the 'Long Walls'. To put Strabo's intuition to the test, 10 geological cores, over 20 meters deep, have been drilled in what is today the Cephissus (Kifisos) plain located between Piraeus and Athens. The stratigraphical study enabled the observed four main stages in the evolution of coastal landscapes in the Piraeus region. (1) During the first stage, 6 700 – 5 500 BC, sea levels in the Mediterranean were considerably lower than today. The hill of Piraeus was not an island and was geographically connected to the mainland. (2) Then from 4 800 – 3 400 BC, sea levels rose, and Piraeus became an island. (3) During the third stage, from 2 800 BC to 1500 BC, sea level rose more slowly, while at the same time massive amounts of sediment were carried down by rivers in the region. This dual phenomenon caused sediments to build up on the Cephissus plain, which led to the establishment of a lagoon environment. (4) Finally, in the fifth century BC, the lagoons were still present. To build the Long Walls, the engineers of the time were therefore forced to fill in these wetlands. Recently, thanks to the Danish Institute at Athens, l’Ecole française d’Athènes, the CNRS, the Paris-Sorbonne University Abu Dhabi and Harokopio University, a scientific program started on the ancient harbours of Piraeus. During the 5th c. BC, Greek engineers built 3 mains harbours around the Piraeus rocky hill: Zea, Mounichia and Kantharos. Off shore cores have been carried out in Zea and Mounichia in order to understand the geoarchaeology of these military basins. Radiocarbon dating and multidisciplinary analyses are in process. They will bring 3 main results: (1) the depth of the harbour basins, (2) any dredging phases, (3) sedimentary rate and spanlife.