Byron Hot Springs in eastern Contra Costa County, California, grew from a long-used Indigenous spring site into one of the state’s notable mineral resorts and later a wartime military installation. Used for centuries by the Bolbone band of Ohlone people, the springs were commercially developed beginning in 1865 and expanded with railroad access. Repeated fires destroyed successive hotels, but the resort flourished in the early 20th century, attracting wealthy visitors to its hot, sulfurous, mineral waters. After closing in 1938, the property became Camp Tracy during World War II, where more than 3,500 Japanese prisoners of war were interrogated, before later reuse and eventual abandonment.