The earliest known built pool is the ‘Great Bath’ structure at Mohenjo-daro (Mound of the Dead), discovered in 1925 in the south of what is now Pakistan. The city, part of the Indus civilisation, was founded between four and five thousand years ago on the flood plain of the mighty River Indus and had at least 35,000 residents. Mohenjo-daro was a planned city, built of fired brick and set on top of a vast mud-brick platform some 1km (approximately 1100 yards) square and 7m (23ft) high. The buildings and streets were laid out on a regular grid and built with walls often 1-2m (3.25-6.5ft) thick for thermal performance, given summer temperatures reaching 55°C (131°F). Each property consisted of one-and two-storey buildings with courtyards containing washing and lavatory facilities. When the Indus River changed its course around 3700 years ago, the civilisation declined and the city was abandoned. The fired brick was so good that the walls were looted to create the nearby railway embankment, which was constructed in the late 19th century.