Where was one of the worst Confederate Civil War prison camps?

"American Civil War Prison Camps were operated by both the Union and the Confederacy to handle the 409,000 soldiers captured during the war, 1861-1865. The Record and Pension Office in 1901 counted 211,000 Northerners who were captured. In 1861-63 most were immediately paroled; after the parole exchange system broke down in 1863, about 195,000 went to prison camps. Some tried to escape but few succeeded"