The Khewra Salt Mine (or Mayo Salt Mine) is located in Khewra, north of Pind Dadan Khan, an administrative subdivision of Jhelum District, Punjab Region, Pakistan, which rises from the Indo-Gangetic Plain. It is Pakistan's largest and oldest salt mine and the world's second largest. It is a major tourist attraction, drawing up to 250,000 visitors a year. Its history dates back to its discovery by Alexander's troops in 320 BC, but it started trading in the Mughal era. The main tunnel at ground level was developed by Dr. H. Warth, a mining engineer, in 1872 during British rule. After independence, the Pakistan Mineral Development Corporation took over the mine, which still remains the largest source of salt in the country, producing more than 350,000 tons per annum of about 99% pure halite. Estimates of the reserves of salt in the mine vary from 82 million tons to 600 million tons.
The Khewra Salt Mine is also known as Mayo Salt Mine, in honour of Lord Mayo, who visited it as Viceroy of India. The mine is a part of a salt range that originated about 800 million years ago, when evaporation of a shallow sea followed by geological movement formed a salt range that stretched for about 300 kilometers (185 miles).
The salt reserves at Khewra were discovered when Alexander the Great crossed the Jhelum and Mianwali region during his Indian campaign. The mine was discovered, however, not by Alexander, nor by his allies, but by his army's horses, when they were found licking the stones. Ailing horses of his army also recovered after licking the rock salt stones. During the Mughal era the salt was traded in various markets, as far away as Central Asia. On the downfall of the Mughal empire, the mine was taken over by Sikhs. Hari Singh Nalwa, the Sikh Commander-in-Chief, shared the management of the Salt Range with Gulab Singh, the Raja of Jammu. The former controlled the Warcha mine, while the latter held Khewra. The salt quarried during Sikh rule was both eaten and used as a source of revenue.
In 1872, some time after they had taken over the Sikhs' territory, the British developed the mine further. They found the mining to have been inefficient, with irregular and narrow tunnels and entrances that made the movement of labourers difficult and dangerous. The supply of water inside the mine was poor, and there was no storage facility for the mined salt. The only road to the mine was over difficult, rocky terrain. To address these problems the government levelled the road, built warehouses, provided a water supply, improved the entrances and tunnels, and introduced a better mechanism for excavation of salt. Penalties were introduced to control salt smuggling.
While working with Geological Survey of India in the 1930s and 1940s, Birbal Sahni found evidence of angiosperms, gymnosperms and insects from the Cambrian period inside the mine.
Khewra Salt Mine is situated in Pind Dadan Khan Tehsil of Jhelum District. Located about 200 km (125 miles) from Islamabad and Lahore, it is accessed via the M2 motorway, about 30 kilometers (20 miles) off the Lilla interchange while going towards Pind Dadan Khan on the Lilla road. The mine is in mountains that are part of a salt range, a mineral-rich mountain system extending about 200 km from the Jhelum river south of Pothohar Plateau to where the Jhelum river joins the Indus river. Khewra mine is about 288 meters (945 feet) above sea level and about 730 meters (2400 feet) into the mountain from the mine entrance. The underground mine covers an area of 110 km2 (43 sq. miles).