We present here the results of three underwater excavation campaigns conducted between 1984 and 1986 on the harbour of Amathus, Cyprus, an exceptional monument of military architecture from the Early Hellenistic era. After a description of the twenty sondages undertaken along the moles, there follows a nomenclature of the blocks employed in the construction, their modules, the typology of bosses for hoisting and placing by a vertical boom crane, which gradually advanced along the mole as it was being built. We describe the nearby quarries in which negatives of the blocks used in the moles were found. The coins have pointed to the attribution of the construction to Demetrius Poliorcetes after his naval victory at Salamis in 306 BC, and the abandonment during the reconquest of the island by Ptolemy I in 294. In the second part, we study the Late Roman wells that were excavated on the north side of the harbour basin, which testify to large changes in sea level.