In the heart of a huge, warped galaxy about 750 million light-years from Earth, a dance is unfolding. And the dancers—two of the largest black holes on record—may be orbiting each other in the closest such pas de deux ever reported, according to a new study. The black holes are separated by just 24 light-years in Galaxy 0402+379, and together contain 15 billion times the mass of our sun. Using four sets of measurements taken by a widespread network of radio telescopes between 2003 and 2015, along with data gathered at optical wavelengths, astronomers discovered that the black holes appear to be circling each other on a 30,000-year cycle, they report today in The Astrophysical Journal. Besides identifying the closest orbiting black holes yet reported, the new study is notable for another reason, the astronomers write: The apparent speed at which these black holes are slowly moving away from one another, as measured from Earth, may be the smallest motion ever discerned. Their apparent separation of just 1 microarcsecond per year (an angle about one-billionth the size of the smallest object visible to the naked eye) is equivalent to the motion earthbound astronomers would measure for a snail creeping across the surface of a planet located 4 light-years from Earth.