Fort McKinley, Double Mine Building, East side of East Side Drive, approximately 125 feet south of Weymouth Way, Great Diamond Island, Portland, Cumberland County, ME

Significance: Fort McKinley attains significance as the largest of Portland Harbor's five military complexes built in the late 19th and early 20th centuries. The fort protected Maine's principal city with one of the most well-defended harbors in the country. The Double Mine Building is the only one of its type built at the fort. It was built as the prototype of the Sewell construction technique. Like the two mine casemates, the Double Mine Building was the location for monitoring and controlling mines in the harbor channels.
Survey number: HAER ME-59-D
Building/structure dates: 1904-1905 Initial Construction