Black holes typically reflect a portion of the ripples in space called gravitational waves, like a bell that doesn't ring purely when struck. Physicists discovered that by precisely shaping these waves to match the black hole's natural resonance, they can make it absorb all the energy, becoming perfectly black and silent during the impact. The energy is stored momentarily.

Then, the black hole releases the energy as a clean, ringing tone. Like the ring of a bell reveals its exact shape, this echo carries detailed information about the black hole's inner structure. Curiously, while in this perfectly absorbing state, the black hole would be completely invisible, detectable only by its gravitational pull on nearby stars. This opens a new way to probe the spacetime curvature around these objects.