Scientists discovered one of the oldest sewers in the world in Mohenjo Daro, near the River Indus in Pakistan. The 4,000 year-old brick-built drainage system can still be viewed today. The present-day standard in Germany, for example, specifies a sewer manhole every 50 metres. In total, therefore, there are now several million manholes, all made of concrete and with different channels and connections. Each manhole base is therefore a one-off.