Falling into a black hole was never going to be fun. As soon as physicists realized that black holes exist, we knew that getting too close to one spelled certain death. But we used to think an astronaut falling past the point of no return—the so-called event horizon—would not feel anything special. According to Einstein's general theory of relativity, no signposts would mark the spot where the chance of escape dropped to zero. Anyone journeying past the horizon would just seem to fall down, down, down into a pit of blackness.