The wealth of the citizens of Lüneburg is based on a salt spring from a salt diapir that lies about 40 meters underneath the surface of the western part of the city. In medieval times salt was traded through the Stecknitz-Delvenau canal to the port of Lübeck and by ship to all the countries of the Baltic Sea. Changes in maritime-trade and a dominance of the Netherlands in shipping the North Sea caused a decrease in the salt production in Lüneburg and a deep economic crisis. The city recovered slowly from the economic depression during the Industrial Revolution. Many refugees streaming into Lüneburg after World War II caused big problems in the city. The economic and living standards were very poor and unemployment high. The crisis was solved by building new industries and residential buildings. As a result of the production of an estimated quantity of 20,000 tons of salt a year over centuries and the natural dissolving of the salt by ground water, the western part of the city became a subsidence area where many buildings have had to be demolished. After different ideas to solve the problem the district was classified as a special development area. It has been renovated and reconstructed over the past few years thanks to public and private donations. Today Lüneburg is a prospering city which has managed the economic change from old industries to the modern service industry.