At extremes of temperature and pressure above a substance’s so-called critical point, the distinction between liquid and gas phases of that substance stops being meaningful, and the substance enters a homogeneous supercritical phase. For many substances, supercritical temperatures and pressures are difficult to achieve, and that’s doubly true if you’re hoping to achieve them under conditions that still allow for visual observation.
Carbon dioxide, however, has a fairly accessible critical point at about 90° F, 1100 psi, and thus supercritical carbon dioxide can and does have fairly routine industrial applications, notably the decaffeination of coffee. But the really cool part is that, at those temperatures and pressures, it’s not too hard to build a pressure vessel from transparent materials that will actually let you get a good look at a supercritical fluid. Which is exactly what Ben Krasnow—who is fast becoming my personal maker hero—has done.