My ebook, Those Who Wait, was inspired by this unique place. My great, great grandfather was a Union guard here.

In late 1861, Federal officials selected Johnson’s Island as the site for a prisoner of war camp to hold up to 2,500 captured Confederate officers. The island offered easy access by ship for supplies to construct and maintain a prison and its population. Sandusky Bay offered more protection from the elements than on other nearby islands, which were also closer to Canada in the event of a prison break. Woods of hickory and oak trees could provide lumber and fuel. The U.S. government leased half the island from private owner Leonard B. Johnson for $500 a year, and for the duration of the war carefully controlled access to the island.
The 16.5-acre (67,000 m2) prison opened in April 1862. A 15-foot (4.6 m)-high (5 m) wooden stockade surrounded 12 two-story prisoner housing barracks, a hospital, latrines, sutler’s stand, three wells, a pest house, and two large mess halls (added in August 1864). More than 40 buildings stood outside the prison walls, including barns, stables, a limekiln, forts, barracks for officers, and a powder magazine. They were used by the 128th Ohio Volunteer Infantry, which guarded the prison.
Among the prominent Confederate generals imprisoned on Johnson's Island were Isaac R. Trimble and James J. Archer (both captured at the Battle of Gettysburg), William Beall, Thomas Benton Smith, Edward "Allegheny" Johnson and Missouri cavalryman M. Jeff Thompson. Prisoners had a lively community, with amateur theatrical performances, publishing, and crafts projects.
After the unraveling of a Confederate espionage ring which had been plotting the seizure of the Great Lakes warship USS Michigan and a mass breakout of prisoners, Forts Johnson and Hill were constructed over the winter of 1864–65. They were not operational until March 1865, in the war's final months, when the prisoner population peaked at 3,200. More than 15,000 men passed through Johnson’s Island until it was closed in September 1865. Wardens lost only about 200 prisoners as a result of the harsh Ohio winters, food and fuel shortages, and disease. Johnson's Island had one of the lowest mortality rates of any Civil War prison. Confederates made many escape attempts, including efforts by some to walk across the frozen Lake Erie to freedom in Canada. A handful of escapes were successful.

After the war, the prison camp was abandoned and control reverted to the owner. Most of the buildings were auctioned off by the Army, and some were razed after falling into disrepair. Efforts in 1897 to turn the island into a resort (as with nearby Cedar Point) failed, and the land was used for farming and rock quarrying. Many lakeside homes have since been built, and is now quite developed with two subdivisions. Most of the Civil War-related sites have since been destroyed and built over.
In 1990 Johnson’s Island was designated a National Historic Landmark. A causeway was built to connect it with the mainland. Only the Confederate cemetery is open to the public. Ground-penetrating radar studies have proved that several graves lie outside its fence. Heidelberg College conducts yearly archeology digs at the prison site.

The Johnson's Island Museum and information center, located on the mainland in Marblehead, Ohio, is dedicated to the history of Johnson's Island and run by the Johnson Island Preservation Society. Opened in 2001, the museum exhibits focus on the island's history as a Civil War POW camp and later uses as a pleasure resort and a quarry. The museum is open on weekends and holidays from Memorial Day through Labor Day.

From: Wikpedia.