Stephanie Dalley, a historian at Oxford University, has revealed that the gardens were probably constructed in the early seventh century BC, 300 miles (433km) north of Babylon in the ancient city of Nineveh, once the centre of the Assyrian empire. Her work also suggests that the gardens' builder was the notorious Assyrian monarch Sennacherib, who was responsible for the destruction of Babylon.
It is already known that Sennacherib built his own gardens. The British Museum possesses a bas-relief sculpture depicting the gardens, immodestly described by the Assyrian king as 'a wonder for all the world'.
The gardens must have been quite literally hanging - trees and other plants clinging to an artificial mountainside with streams filled with water lifted by an Archimedes-style water screw - four centuries before Archimedes' invention. Sennacherib even boasted how he had designed the screw himself.
Dr Dalley believes that the earliest references to the hanging gardens being in Babylon must be regarded as thoroughly unreliable.
The oldest source - a third century BC Greco-Babylonian writer called Berosus - wrote his account three centuries after any Babylonian hanging garden would have fallen into disuse after the diversion of Babylon's royal palace water supply by the Persians, who conquered the city in 539BC.
Likewise, the second century BC Levantine writer Antipater, one of the first to list the seven wonders, lived long after the gardens - whether in Babylon or Nineveh - were destroyed. Nineveh itself was razed by the Babylonians in 612 BC.
Dr Dalley also points out that contemporary commentators in ancient Babylon failed to mention any gardens. King Nebuchadnezzar proudly listed all his achievements - but no gardens.
Dr Dalley cites the first-century BC Graeco-Sicilian historian, Diodorus Siculus, as saying that the hanging gardens of Babylon were built by an Assyrian king who conquered the city. The first-century AD Roman historian, Quintus Curtius Rufus, also maintained that the hanging gardens were built by an Assyrian king.
And Strabo, the first-century BC Greek geographer, described the gardens in virtually the same terms used by Sennacherib to record his own gardens at Nineveh, and also described a screw system for raising water.
The seven wonders - as recorded by Antipater - were the pyramids of Egypt, the hanging gardens of Babylon, the statue of Zeus at Olympia, the temple of Artemis at Ephesus, the mausoleum of Halicarnassus, the colossus of Rhodes and the great lighthouse of Alexandria.
Dr Dalley's discovery will have considerable implications for archaeological reconstruction work now going on amid the ruins of ancient Babylon, located in Iraq, 50 miles (72km) south of Baghdad. The Iraqi president, Saddam Hussein, has long been committed to restoring Babylon's ruins to their ancient glory, including the rebuilding of the hanging gardens.