In use since Roman times, today the salt pans are the source of 18,000 metric tons (20,000 tons) of salt a year.
Salt pans have only been in use since Medieval times.
mnli hypothesis: Salt pans have only been in use since Medieval times. premise: In use since Roman times, today the salt pans are the source of 18,000 metric tons (20,000 tons) of salt a year.