The mass of a black hole is so tightly packed that its enormous gravitational field devours every celestial body in its vicinity. This immense gravity generates a boundary around the black hole, the event horizon, that not even light is fast enough to escape. Inside the event horizon, anything is inevitably drawn towards the center, where the entire mass of the black hole is compressed in a single point. A mind-boggling concept, right at the frontier of our understanding. Not even the most advanced theory can yet fully describe what happens inside the event horizon.