While he's best known for designing the Woolworth Building and other Beaux Arts structures, architect Cass Gilbert was also behind what was the United States' largest military base through World War II: The Brooklyn Army Terminal, which opened its doors for Open House New York this past week.

Construction on the four million square foot Brooklyn Army Terminal started in 1918 and ended the next year, paving the way for it to have 56,000 military and civilian workers at its peak, with three million troops and 37 million tons of military supplies entering and leaving the quarters. According to the terminal's history, "Arguably the most famous soldier to deploy from BKLYN Army Terminal was Elvis Presley. He greeted fans and a dozens of photojournalists in September of 1958 when he shipped off from Brooklyn to Germany." Elvis probably took the LIRR to get there, and trains ran through its atrium.

Located in Sunset Park, the Brooklyn Army Terminal was also the largest concrete structure in the world at one point. Now owned by NYC, the building houses commercial tenants, like Chocolatier Jacque Torres (for chocolate manufacturing); sustainable furniture brand Urban Green; and Lee Spring, which makes mechanical springs.