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Berlin Wall , fortified wall surrounding West Berlin, built in 1961 and maintained by the former German Democratic Republic (GDR), commonly known as East Germany, until 1989. The Berlin Wall was a highly visible symbol of the Cold War, the post-1945 struggle between the Union of Soviet Socialist Republics (USSR) and its allies, including East Germany, and the United States and its allies. At the end of World War II in 1945, the city of Berlin was completely surrounded by territory occupied by Soviet forces. This territory officially became the country of East Germany in 1949. The city of Berlin itself was partitioned into East Berlin and West Berlin. West Berlin was occupied by British, French, and United States forces and was supported by the Federal Republic of Germany, commonly known as West Germany. Between 1949 when East Germany was established and the middle of 1961, at least 2.7 million people fled East Germany, more than half of them through West Berlin. Compare...

Although the first Ming walls were built of earth in the traditional manner, by the 16th century the work had become much more elaborate and was done in stone by professional builders paid in silver. Bit by bit, in response to Mongol challenges, the Ming heavily fortified the region around the capital at Beijing. Other areas were protected with shorter walls or forts, or had no defenses at all. Wall building and repair continued until the Ming dynasty fell to the Qing dynasty in 1644. By this time, the walls formed an incomplete and uneven network. The eastern end was at Qinhuangdao, in Hebei Province on the gulf of Bo Hai, while the western extreme was near Jiayuguan in Gansu Province. The walls spanned mountainous terrain, conforming to the territory’s numerous peaks and valleys. They included inner walls and outer walls, and some stretches had watchtowers placed at regular intervals so that alarm signals could be passed between them i...

A Luis Alvarez   1911 – 1988. The iridium layer, dinosaur death by meteorite impact, and subatomic particle discoveries. André-Marie Ampère  1775 – 1836. Discovered that wires carrying electric current can attract and repel magnetically; founded electromagnetic theory. Anaximander  c. 610 BC – c 546 BC. An ancient scientific revolution: the first person in history to recognize that our planet is free in space and does not need to sit on something. Mary Anning 1799 – 1847.  Ancient animals, fossils, and paleontology: discovered the first complete specimen of a plesiosaur; deduced the diets of dinosaurs. Archimedes   c. 287 BC – 212 BC. Founded the sciences of mechanics and hydrostatics, calculated pi precisely, devised the law of exponents, created new geometrical proofs, invented numerous ingenious mechanical devices, and more. Aristarchus   c. 310 BC – c. 230 BC. Promoted the idea that the earth follows a circular orbit around...