and the oldest free standing structures are the Megalithic Temples of Malta. They start in the 5th millennium BC, though some authors speculate on Mesolithic roots. One of the best-known prehistoric sites is Stonehenge, part of the Stonehenge World Heritage Site which contains hundreds of monuments and archaeological sites. Monuments have been found throughout most of Western and Northern Europe, notably at Carnac, France. The large mound tomb at Newgrange, Ireland, dating to around 3200 BC, has its entrance marked with a massive stone carved with a complex design of spirals. The mound at nearby Knowth has large flat rocks with rock engravings on their vertical faces all around its circumference, for which various meanings have been suggested, including depictions of the local valley, and the oldest known image of the Moon. Many of these monuments were megalithic tombs, and archaeologists speculate that most have religious significance. Knowth is reputed to have approximately one third of all megalithic art in Western Europe. In the central Alps, the Camunni made some 350,000 petroglyphs: see Rock Drawings in Valcamonica. Bronze Age During the 3rd millennium BCE, the Bronze Age began in Europe, bringing with it a new medium for art. The increased efficiency of bronze tools also meant an increase in productivity, which led to a surplus — the first step in the creation of a class of artisans. Because of the increased wealth of society, luxury goods began to be created, especially decorated weapons. Examples include ceremonial bronze helmets, ornamental ax-heads and swords, elaborate instruments such as lurer, and other ceremonial objects without a practical purpose, such as the oversize Oxborough Dirk. Special objects were made in gold; many more gold objects have survived from Western and Central Europe than from the Iron Age, many mysterious and strange objects ranging from lunulas, apparently an Irish speciality, the Mold Cape and Golden hats. Pottery from Central Europe can be elaborately shaped and decorated. Rock art, showing scenes from the religious rituals have been found in many areas, for example in Bohuslän, Sweden and the Val Camonica in northern Italy. In the Mediterranean, the Minoan civilization was highly developed, with palace complexes from which sections of frescos have been excavated. Contemporary Ancient Egyptian art and that of other advanced Near Eastern cultures can no longer be treated as "prehistoric". Iron Age The Iron Age saw the development of anthropomorphic sculptures, such as the