Stonehenge, towering in proud seclusion over the grassy Salisbury Plain in South England, is probably the world's most famous prehistoric monument. Attended by hundreds of thousands of people annually, feeding the minds of generations of historians and archaeologists, Stonehenge has become a synonym for ancient mystery. Huge sarsens - vertically placed blocks of dense gray limestone reaching 13 feet in height, roughly fringed and connected by horizontal slabs - are the first thing that strikes the eye of visitors. From the northeast, the megalithic circle is accessed by an alley with shallow ditches on both sides. Having passed the Heel Stone (a free-standing sarsen), you cross two little visible earth ramparts separated by a ditch and forming a double ring around Stonehenge. Inside the megalithic circle there is another circle of smaller stones called "blue stones". In the circle of "blue stones" there is a number of megaliths in the form of horseshoeshoeshoes, turned in the northeast