In 1994, the Retsof Salt Mine was the largest salt mine in North America, and the second largest in the world. Three hundred people worked within the 6,000 acres (24 km2; 9.4 sq mi) of excavated space, 1,000 feet (300 m) below ground, extracting salt from a natural deposit for use as road salt, table salt, and in industry. In March 1994, however, the ceiling in one of the large underground chambers collapsed, the first of a series of effects caused by groundwater entering the salt deposit, which had been dry for all of the 110 previous years of mining at the site. Over the next 21 months, the mine cavities collapsed and filled with water. Mining operations scrambled to work the accessible areas before the spreading flood, until operations were suspended when the mine was fully filled with water, in 1995. The effect of filling all this space lowered the aquifer, leaving many drinking water wells dry, and led to surface subsidence, even sinkholes 200 feet (61 m) wide, damaging structures and highways. 8 feet (2.4 m) or 9 feet (2.7 m) of additional subsidence is expected to take place over the next century