  Materials modelling and processing require experiments to visualize and
quantify how external excitations drive the evolution of deep subsurface
structure and defects that determine properties. Today, 3D movies with ~100-nm
resolution of crystalline structure are regularly acquired in minutes to hours
using X-ray diffraction based imaging. We present an X-ray microscope that
improves this time resolution to <100 femtoseconds, with images attainable even
from a single X-ray pulse. Using this, we resolve the propagation of 18-km/s
acoustic waves in mm-sized diamond crystals, and demonstrate how mechanical
energy thermalizes from picosecond to microsecond timescales. Our approach
unlocks a vast range of new experiments of materials phenomena with intricate
structural dynamics at ultrafast timescales.
