Fort Jefferson is a historic fortress located on Garden Key in the Dry Tortugas National Park, near Key West, Florida, USA. The fort was built in the mid-19th century to protect the Gulf of Mexico from foreign invasion and to secure shipping lanes. It was named after President Thomas Jefferson.

Fort Jefferson is made of over 16 million bricks, making it one of the largest masonry structures in the world. The fort is hexagonal in shape and covers over 16 acres of land. It has walls that are 40 feet high and over 8 feet thick.

Fort Jefferson was used as a military prison during the Civil War, with famous prisoners such as Dr. Samuel Mudd, who was convicted of conspiring with John Wilkes Booth in the assassination of President Abraham Lincoln.

Today, Fort Jefferson is a popular tourist attraction, and visitors can take guided tours of the fort and learn about its history. The Dry Tortugas National Park is only accessible by boat or seaplane, making it a remote and peaceful destination for outdoor enthusiasts and history buffs alike.