The Great Bath of Mohenjo-daro was made watertight using a thick layer of:

The Great Bath is one of the most famous structures of the Indus Valley Civilisation. It is located in the citadel area of Mohenjo-daro, in the Larkana district of Sindh, in present-day Pakistan. It is often described as the earliest public water tank in the ancient world.

The Great Bath is a large rectangular tank, about 12 metres long, 7 metres wide and 2.4 metres deep. It is made of finely fitted burnt bricks, laid on edge and set in gypsum mortar. Flights of steps at the northern and southern ends lead down into the tank. To make it watertight, the builders lined the floor and sides with a thick layer of bitumen, a black, sticky, natural tar, behind the brick lining. Water was probably supplied from a large well in an adjoining room, and an outlet in one corner led to a large drain to empty the tank.

The Bath was surrounded by a corridor and rooms on all sides. In one of the buildings nearby, there were small rooms, possibly for changing clothes. Scholars believe that the Great Bath was used for ritual bathing, which has long been important in Indian religious practice.

Mohenjo-daro means "mound of the dead" in Sindhi. It was discovered in 1922 by R.D. Banerji and lies on the right bank of the Indus river. Other important finds from Mohenjo-daro include:

The "Pashupati" seal, showing a figure surrounded by animals.

A piece of woven cotton cloth.

Mohenjo-daro was declared a UNESCO World Heritage Site in 1980. The Harappans were famous for their town planning, with grid-pattern streets and covered drains.

Why not the other options?

• ✗Lime plaster was not the main waterproofing material of the Great Bath.
• ✗Clay alone would not make the tank watertight.
• ✗Gypsum was used in the mortar between bricks, not as the main waterproof layer.