The Khewra Salt Mine (or Mayo Salt Mine) is located in Khewra, in Jehlum district, Punjab, Pakistan. It is Pakistan's largest and oldest salt mine and the world's second largest. Its history dates back to its discovery by Alexander's troops in 320 BC, but it started trading in the Mughal era. Further work was carried out during the British Rule and after partition of India and Pakistan, the government took over it’s possession under mines division. It is a geological wonder and a must visit, if you like to visit unique places. I haven’t visited it yet, but hope to set my foot on it someday.
Further details about the Khewra Salt Mine can be found at Wikipedia.