There seems to be strong factual basis for the other wonders of the ancient world, but the Hanging Gardens of Babylon may be one wonder of the ancient world that never was.
It could simply have been the luxurious appearance of a greenery-covered ziggurat that led writers to claim that a hanging garden existed.
What Ancient Writers Say It Was Like
Diodorus Siculus described the hanging gardens as a "series of superimposed terraces of reducing size, rising to a height of 75 feet." [H.W.F. Saggs "Civilization Before Greece and Rome," p. 55.] Strabo said it was much larger, with a a square base with each side 400 feet long. The water could have been brought from the Euphrates River to the garden by irrigation and a series of buckets or pulleys.
Who Built Them? When?
The Hanging Gardens are said to have been built in the sixth century by Nebuchadnezzar for his wife or in the ninth century, by the Assyrian Queen Semiramis. Hanging probably meant the gardens were overhanging or terraced.
Dr. Stephanie Dalley, of Oxford University's Oriental Institute (according to The biggest wonder about the Hanging Gardens of Babylon? They weren't in Babylon, an article in the UK Independent from May 6, 2013), argues that the Hanging Gardens were in Ninevah, not Babylon, and were built by the Assyrian monarch Sennacherib.