A pressing matter

Under high pressures, some of the molecules and elements most familiar to us - water, hydrogen, oxygen and iron - behave in surprising ways. Inert elements can become reactive, insulators can become superconductors, and unexpected complexity appears in the structures of materials. Even substances as familiar as water and hydrogen behave in surprising ways. Recent high-pressure experiments are therefore providing crucial tests of condensed-matter theory, with implications spanning the physical and biological sciences. (U.K.)