This ginormous Back Bay townhouse—8,000 square feet! 7 bedrooms! 7.5 bathrooms!—is so elegantly mysterious it has no address via its listing, only a nod to a city block that reads like an entry in Burke's Peerage: Marlborough, between Dartmouth and Clarendon. Aside from the sheer enormity of the house and the beauty present in the apparently original detailing, it also has suitably exclusive flourishes like an au pair suite and a guest one, both with separate entrances. Yours for $8,500,000.