An event horizon is the boundary around a black hole beyond which nothing, not even light, can escape. It is not a physical surface but a one-way membrane in spacetime: cross it, and every possible path leads inward toward the singularity. The radius of the event horizon, called the Schwarzschild radius, depends only on the black hole's mass. For an object the mass of the Sun it would be about three kilometers across. The Event Horizon Telescope has now imaged the glowing ring of light just outside two of them, the supermassive black holes in M87 and at the center of the Milky Way.