Nanostructuring on length scales corresponding to phonon mean free paths provides control over heat flow in semiconductors and makes it possible, in principle, to engineer their thermal properties. However, this is currently not feasible because there is no general description for heat flow in 3D nanostructured semiconductors. In recent research, STROBE scientists used short wavelength extreme ultraviolet beams to study heat transport in a silicon metalattice with deep nanoscale features. They observed dramatically reduced thermal conductivity relative to bulk—about x50 times less than current model predictions. To explain this, they developed a new predictive theory that incorporates the idea that heat-carrying lattice vibrations can behave like a fluid—spreading out instead of just moving ballistically in straight lines. Moreover, this new theory of heat transport can be used to predict and engineer phonon transport in many other 3D nanosystems including nanowires and nanomeshes, that are of great interest for next-generation energy-efficient devices.