The Brooklyn Navy Yard is nearly half the size of Prospect Park, but was, until very recently, one of the least accessible areas to the general public in the borough. But with the opening of Building 77 the public will be welcomed for the first time into one of the complex’s most formidable buildings: a hulking 1 million-square-foot block of concrete that’s considered the heart of the entire Brooklyn Navy Yard redevelopment. The companies leasing space in Building 77 are poised to bring in 3,000 new jobs, setting the complex well on its path of creating 17,000 jobs by 2020. David Ehrenberg, president and CEO of the Brooklyn Navy Yard Redevelopment Corporation, the nonprofit overseeing the yard’s rehabilitation, says the goal is to create the largest job expansion the Navy Yard has seen since World War II, when some 70,000 people—or .02 percent of the American workforce—labored within its confines. In its heyday, Building 77 served as a Navy storage warehouse and offices for the naval high command for the North Atlantic fleet and the commandant of the yard. A major part of the building’s $185 million rehab consisted of removing 3 million pounds of concrete from the building and installing 50,000 square feet of windows. It’s that act, Ehrenberg says, that transformed the building from a warehouse to an active manufacturing facility.