The Haddaj Well is an outstanding example of ancient engineering. It reportedly dates back to the Babylonian presence during the 6th century BC, and at 18m across is one of the largest historic wells in the world. It was filled in when the town was abandoned in the 5th century BC, and was said to have been uncovered by a local Jewish man, Suleiman Al Gonaim. The current wheels and pulleys date from improvements made by King Saud in the 1950s.
To get here, exit off Route 15 at the decorated roundabout and follow the brown signs for Haddaj Well. It's worth a visit if you're in the area.