The event horizon of a black hole should be impossible to study. Its the point of no return, the boundary where gravity grows so strong that not even light can escape, so by definition nothing can carry word of it back to us. Yet a team of scientists have found a way to reach it and found a hidden signal, a faint trace, never read before, carrying information from the very edge of the horizon in the instant before it formed. From it they measured the new black hole's spin and surface gravity, and opened a fresh way to test whether Einstein's theory survives in the most extreme gravity there is.