Supercritical water, a mysterious state where water behaves like both a liquid and gas, has long puzzled scientists.

Researchers in Germany used terahertz spectroscopy and powerful simulations to finally debunk a key theory—that water molecules form hydrogen-bonded clusters in this state. Building a specialized high-pressure cell, the team showed that supercritical water behaves much like gas, lacking the long-lasting hydrogen bonds found in liquid water. Simulations confirmed that interactions between molecules are fleeting and disordered.