First things first: The Octagon, on Roosevelt Island, is one of the elite new luxury rental listings on Roosevelt Island -- which is a distinction that increasingly means something. But The Octagon is also more than that -- something simultaneously both more futuristic and more historic than the average Manhattan luxury rental, and another sign of Roosevelt Island's rise as a neighborhood worth watching on the Manhattan real estate scene. The Octagon New Yorkers know today -- a 500-unit luxury rental building that's also one of the greenest green rental buildings in New York City -- was built in 2006, around a (yes) octagonal building with a much longer story behind it. The octagon that forms the hub of The Octagon dates back to 1841, and was built then by Alexander Jackson Davis as a blue octagonal entrance building to what was known then as the New York City Lunatic Asylum (Today, of course, we no longer say "lunatic asylum," and instead use the term "Port Authority Bus Terminal.") That the space has since become one of the most luxurious rental listings on Roosevelt Island -- after 55 years of service as a 19th-century mental hospital, if you're just joining us -- is notable enough. But with the installation of a new, energy-efficient fuel cell that makes The Octagon the first green apartment building in NYC and New York State to be so-powered, The Octagon also has a claim to being one of the greenest green rentals in Manhattan.