A desert island adrift in the Gulf of Mexico contains a massive and marooned structure with a contagious past; experts investigate and uncover a desolate military fortress turned prison and plagued by a deadly disease. Stories: 1) Fort Jefferson, Dry Tortugas, Florida; 2) Bankya Residence, Sofia, Bulgaria; 3) Bunker Valentin, Bremen, Germany; 4) Cockatoo Island, Sydney, Australia