Not all artists leave a signature. Last summer, somebody grabbed the city’s attention by patching a hole in a crumbling Fort Point wall with colorful Lego pieces. This spring, NPR tracked down the identity of that somebody: Nate Swain, a former landscape architect who’s been responsible for other guerilla public art across town. Such installations include Barnacle City, two miniature metropolises made from scraps of brick and Belgian blocks that are visible only at low tide in the Fort Point Channel and the Charles River. Says Swain: “I like to poke fun at or highlight urban layout and design problems in Boston that most people choose to ignore.” Mission accomplished.