2. Man-Made Caves of Salt

3. An enormous salt mine has been operating beneath Detroit for more than a century. Click here to see a drawing of an 1896 mine operation.

4. This bed of salt is hundreds of millions of years old and contains trillions of tons of salt.

5. Detroit began using rock salt on icy roads in 1940 and began selling salt to other cities for the same use. Click here to see some other uses for salt.

6. The mine spreads over 1,400 acres under Detroit and its suburbs.

7. Electric trucks do much of the work. Click here to see a conveyorClick here to see a drill truckClick here to see a shovel

8. The Process of Salt Mining

9. The miners use a room-and- pillar method to get the salt. The black blocks represent pillars of salt. A room and pillar mine. Photo courtesy Detroit Salt Company The blue grid represents the tunnels where the salt is mined.

10. Fifty miles of road carry construction equipment, trucks, and cars, which had to be taken apart to fit down the shaft. Photo courtesy Kansas Geological Survey

11. If the mine goes out of business, it could be used to store important items or become a museum. The Historic Wieliczka salt mine near Krakow, Poland, houses a fascinating museum.

12. The End Click to replay slide show

13. An 1896 Mining Operation Click here to return to the previous slide.

14. Early uses for salt included storing meat and fish and making ice cream. Click here to return to previous slide.

15. Giant chainsaw trucks cut grooves in the mine wall and huge conveyors move the salt blocks. Underground salt mine conveyor Photo courtesy of Kansas Geological Survey Click here to return to previous slide.

16. Drill trucks bore holes in the wall for dynamite. Salt mine drill and transport. Photo courtesy of Kansas Geological Survey Click here to return to previous slide.

17. Shovel trucks scoop up rock salt, which then goes into dump trucks. Click here to return to previous slide.