Tucked among the sea of warehouses strewn across the Brooklyn Navy Yard is an unsuspecting, secret beer aging bunker lined with thousands of barrels of experimental varieties of beer. Located in the shipyard’s Building 269, home of the historic and quirky Agger Fish warehouse, this is Brooklyn Brewery’s Barrel Room, the most delicious of R&D spaces.

This hidden testing and tasting facility stores up to 2,000 barrels of aging beer, busy seeping in the flavors of the wood and the different spirits that were previously kept in the barrels—mezcal, cognac, bourbon, red wine, and so on. Bourbon barrels have long been the go-to medium for aging beer, but more unique spirits are becoming increasingly popular: A mezcal barrel might infuse a smoky flavor into the beer, while a wine barrel may lend a fruity taste. 

Aging is vital to the beer-making process. As the brew ferments, its sugar and yeast interact to produce CO2 and alcohol as byproducts. But this alcoholic blend is unpalatably flat, and requires weeks or months of aging for the brew to absorb the carbon dioxide, giving the beer its fizz. At any given time a variety of one-off special project beers are being aged. At the end of the process, brewery staff tests the beer and decides which will go on to become special releases, and which will never officially see the light of day.