Notorious, a hell on earth, a cesspool, a death camp, and infamous have all been used by prisoners and critics to describe Andersonville Prison, constructed to house Union prisoners of war in 1864, and all descriptions apply. Located in Andersonville, Georgia and known colloquially as Camp Sumter, Andersonville only served as a prison camp for 14 months, but during that time 45,000 Union soldiers suffered there, and nearly 13,000 died. Victims found at the end of the war who had been held at Camp Sumter resembled victims of Auschwitz, starving and left to die with no regard for human life.
Rumors about the horrors of Andersonville were making the rounds by the summer of 1864, and they were bad enough that during the Atlanta campaign, Union General William Tecumseh Sherman gave orders for a cavalry raid attempting to liberate the prisoners there. The Union cavalry were repulsed by Southern militia and cavalry at that point, and even after Sherman took Atlanta, the retreating Confederates moved under the assumption that the Union would target Andersonville yet again.
Before the end of the war, the Confederates were moving prisoners from Andersonville to Camp Lawton, but by then, Andersonville was already synonymous with horror.