There are no other walls like these. They are different from Stonehenge, different from the Pyramids of the Egyptians and the Maya, different from any of the other ancient monolithic stone-works. The stones fit so perfectly that no blade of grass or steel can slide between them. There is no mortar. They often join in complex and irregular surfaces that would appear to be a nightmare for the stonemason. Scientists speculate that the masonry process might have worked like this: after carving the desired shape out of the first boulder and fitting it in place, the masons would somehow suspend the second boulder on scaffolding next to the first one. They would then have to trace out a pattern on the second boulder in order to plan the appropriate jigsaw shape that would fit the two together. In order to make a precise copy of the first boulder's edges, the masons might have used a straight stick with a hanging plum- bob to trace its edges and mark off exact points for carving on the second