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.
Supercritical water is used for eco-friendly disposal of toxic waste: it seeps like a gas and dissolves like a liquid.