The Secrets of Sumer: "Unearthing the Lost Cities of Mesopotamia" - a team of archaeologists discovers an ancient map leading to a hidden civilization, unveiling advanced technology and forgotten deities that challenge everything they thought they knew about human history. Sumerians were among the first known cultures to develop many benchmarks that are used to define a "civilization". They are credited with the establishing codes of law, the plow, the sailboat, and a lunar calendar. They also developed a numerical system, based on the number 60 that is still used to measure seconds and minutes. However, probably the most famous legacy is their writing system.The Sumerians devised one of the earliest writing systems known as cuneiform, or wedge-shaped symbols. The earliest known cuneiform inscriptions were found in the lower Tigris-Euphrates Valley in what is now southeastern Iraq and date from about 3,000 BC. Writers made the symbols by pressing a pointed instrument called a stylus into wet clay tablets.