Construction was begun on the Brooklyn-Battery Tunnel in 1940. Initially, Robert Moses had pushed for a bridge in the same place, but those plans were scrapped by the Dept. of War citing both security concerns (the bridge would be close to Brooklyn's Navy Yard) and citizen's outcry to preserve the Battery, which was largely undeveloped at the time (an unstated reason was Moses' feud with President Roosevelt.) Instead of a bridge, a tunnel was built to connect the two boroughs. Four towers ventilate the two-mile-long strip of underground highway, refreshing air every 90-seconds.

Brooklyn approach to the Brooklyn-Battery Tunnel, 1950. Conceived in the late 1930s by city officials, this project was one that master planner Robert Moses was actually not a proponent (and complained about the resulting tunnel for decades). Moses wanted to construct a bridge between lower Manhattan and Brooklyn but city officials, including Mayor LaGuardia, and President Roosevelt advocated for the tunnel (with FDR citing issues of national security in the construction of a bridge as well as his dislike for Moses). Construction on the tunnel began in 1940, with FDR attending the groundbreaking ceremony, but was waylaid by World War II. Completed and opened to traffic in 1950 it is the longest continuous underwater road in North America. In 2010 it was renamed (officially, though not colloquially) the Hugh L. Carey Tunnel, for the former New York State Governor.

The completion of the Brooklyn-Battery Tunnel in May, 1950 marked the end of 10 years of planning and engineering, overcoming back-breaking labor, war, and financial difficulty that stood in its way. At the time the nation’s longest most modern underwater vehicular tunnel, the Brooklyn-Battery tunnel had about as turbulent a history as any of New York’s infrastructure. It was the most expensive two-mile bit of highway ever built at the time - costing more than $40,000,000 a mile. A connection between Brooklyn and Manhattan was first conceived of in 1929. Initially, a graceful, double-unit suspension bridge was to connect the boroughs, featuring a ramp leading down to Governor's Island at the apex. Unfortunately, various groups objected against its potential to disfigure the Battery Park Landscape, and President FDR cited security concerns. In the end, the War Department barred the construction of the bridge in 1939. Funding dwindled on the endeavor to drill a two-mile tunnel between Battery Park and Brooklyn until the Reconstruction Finance Corporation offered $57,000,000 in loans towards its construction. Ground broke on October 28th, 1490, with President Franklin D. Roosevelt on hand for the momentous occasion. Major construction work began in 1941 and for nearly two years, it slowly marched forward. On the Manhattan end, a hole 24-feet in diameter was carved through solid rock which began 2,800 feet under the surface of the river. The Brooklyn side was challenged with softer rock to drill through at about 1,000 feet from the entrance at Hamilton Avenue and Van Brunt Street. The United States entry into World War II put an end to construction on the tunnel, leaving both materials and manpower becoming scarcer every day. In fact, progress on the tunnel came to a standstill towards the end of 1942. For three years the whole project lay idle, only the throne of pumps on the Brooklyn side signifying that the tunnel job was not forgotten. Things began humming again at the end of the war in 1945. Water was pumped out, concrete shields were removed, and men came back to work. It took another five years for the tunnel to be completed and acclaimed as a modern marvel. Fully illuminated, with fresh air exchanged every 90 seconds, the tunnel was opened with great fanfare.