Hanging Gardens of Babylon
Iraq

Why these ancient Seven Wonders gardens were special
The Hanging Gardens of Babylon are the most controversial of all the Seven Wonders because some archeologists doubt they ever existed. After exploring the ruins firsthand and reading extensively, I believe they did exist, but on a less grand scale than widely thought. 

Why were the Hanging Gardens of Babylon built? 
The most popular account is that King Nebuchadnezzar II (of biblical fame) constructed this wonder in the 6th century BC to please his wife. Babylon's flat desert-like landscape made her homesick for the verdant mountains of Media where she was reared. So, the king built an artificial, terraced hill lushly cultivated with trees and flowering plants.
Others say that a powerful Assyrian queen built the Hanging Gardens of Babylon for her own pleasure. Others believe it was conceived much earlier by an Assyrian king. Yet other experts go so far as to think that the Hanging Gardens existed in Nineveh, not Babylon.

Interesting tidbits about 
the Hanging Gardens of Babylon
 Ancient accounts of the Hanging Gardens of Babylon are scarce. Although several ancient Greek writers describe them, none ever saw them. And, "hanging gardens" was never mentioned on any of the numerous clay-tablet records that archeologists excavated from Babylon.
What remains of what is widely believed to be the Hanging Gardens site is mainly red brick rubble.
Substantial water was needed for irrigation because water evaporates quickly in Babylon's arid environment.
Devising a workable watering system was more of a challenge than erecting the gardens. The water had to be diverted from the Euphrates River and raised to the level of the holding tank above the Hanging Gardens.
And, the Hanging Gardens could collapse if water were absorbed by the brick columns and foundation supporting the gardens. 
The estimated lifespan of the Hanging Gardens of Babylon was only a century or so, likely due to neglect. This in turn caused water damage and clay brick deterioration. 
The gardens "overhang" the terraces rather than "hang" from ropes. A long-ago translator apparently erred.
Babylon was one of the world's most impressive cities with its magnificent palace, temples, walls and gates. The 5th century BC Greek historian Herodotus wrote, "Babylon surpasses in splendor any city in the known world." It's possible that Babylon was originally on the Seven Wonders list and a Medieval scribe took it upon himself to substitute the Hanging Gardens for it.