Modern sewer systems are more complex, leading to sewage treatment plants where the water is treated via filtration and addition of various chemicals to disinfect and remove contaminants before it's returned to nature. Modern environmental engineers work on solutions to issues like pollution reduction and cleanup, energy consumption and emissions, land erosion, water treatment and waste management in an effort to properly manage and maintain the quality of our soil, water and air. They used the force of gravity to move water downhill via manmade conduits constructed at a steadily falling incline. It sent water to smaller castella, from which it flowed via masonry conduits or pipes to feed fountains, baths, public drinking basins and sometimes even private residences. By a few hundred years B.C.E., the Greeks had sewer systems that transported rain and wastewater to collection basins that irrigated and fertilized fields. Between 2000 and 4000 B.C.E., the Mesopotamian Empire (modern-day Iraq), Mohenjo-Daro (modern-day Pakistan), Egypt, the island of Crete and the Orkney Islands in Scotland already had drainage systems -- and, in some cases, indoor sanitation facilities. Sewer systems fit the bill by transporting large amounts of human excrement away from populated areas, and they have been evolving for thousands of years.