Water in a kettle, as it boils, clearly separates into liquid and steam. But under enormous pressure, the boundary vanishes: the substance enters a supercritical state — neither one nor the other, but something in between.

Black holes can do this too. In the 1970s, Stephen Hawking and Jacob Bekenstein discovered that they have a temperature and entropy (a measure of disorder). Now physicists have applied to them a method originally developed for fluids. It uses imaginary numbers to reveal hidden transitions, like a photographic negative slowly filling with detail. In this invisible realm, a black hole displays not one supercritical zone but three: liquid-like, gas-like, and completely indistinguishable.

Previously, that region was thought to be unique. This new finding helps us understand the quantum nature of gravity and possibly the structure of neutron stars.