construction of housing for up to 3,000 workers. Gypsum was processed on site, whereas beginning in 1887, after the construction of a line connecting Wapno and Nakło to Gniezno, salt was transported by rail to Nakło for processing . On September 28, 1977, the Tadeusz Koscziusko saltmine underneath the town center collapsed, forcing the evacuation of around 1000 residents. Wapno has never recovered economically from the loss of mining jobs after the accident, which has never been fully explained.A 2014 study found that a likely cause of the accident was exploitation of resources at the tenth and lowest level of the mine, along with preparatory work at the eighth and ninth levels, which study authors called a "mistake," as room and pillar mining of rock salt tends to lead to the accumulation of fractures, fissures, and leaching of liquified sand and soil that lead to subsidence. References External links Official site (in Polish, includes photos) Villages in Wągrowiec County