The Great Bath is one of the best-known structures among the ruins of the Indus Valley Civilization, excavated at Mohenjo-daro in Sindh region of undivided India (now in Pakistan). Archaeological evidence indicates that the Great Bath was built in the third millennium BC, soon after the raising of the "citadel" mound on which it is located.
The Great Bath of Mohenjo-Daro is called the "earliest public water tank of the ancient world". It measures approximately 12 metres (40 ft) by 7 metres (23 ft), with a maximum depth of 2.4 metres (8 ft). Two wide staircases, one from the north and one from the south, served as the entry to the structure. The sloping floor leads to a small outlet at the southwestern corner of the tank, connecting corbelled arch drain, which led the used water out of the bath.
The floor of the tank was watertight due to finely fitted bricks laid on edge with a gypsum plaster, and the side walls were constructed in a similar manner. To make the tank even more watertight, a thick layer of bitumen (waterproof tar) was laid along the sides of the pool and presumably also on the floor.
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