From the World Heritage inscription:
The Palmeral of Elche, a landscape of groves of date palms, was formally laid out, with elaborate irrigation systems, at the time the Muslim city of Elche was erected, towards the end of the tenth century A.C., when much of the Iberian peninsula was Arab. The Palmeral is an oasis, a system for agrarian production in arid areas. It is also a unique example of Arab agricultural practices on the European continent. Cultivation of date palms in Elche is known at least since the Iberian times, dating around the fifth century B.C.
This is one of the most unique World Heritage Sites I have ever visited. The old city is basically build within a palm forest. Everywhere you go are palm trees and palm gardens. The palm forest was originally created by the Arabs when they settled in Elche to remind them of home.
View my complete list of UNESCO World Heritage sites.