  Femtosecond high-intensity laser pulses at intensities surpassing $10^{14}
\,\text{W}/\text{cm}^2$ can generate a diverse range of functional surface
nanostructures. Achieving precise control over the production of these
functional structures necessitates a thorough understanding of the surface
morphology dynamics with nanometer-scale spatial resolution and
picosecond-scale temporal resolution. In this study, we show that individual
XFEL pulses can elucidate structural changes on surfaces induced by
laser-generated plasmas, employing grazing-incidence small-angle x-ray
scattering (GISAXS). Using aluminum-coated multilayer samples we can
differentiate between ultrafast surface morphology dynamics and subsequent
subsurface density dynamics, achieving nanometer-depth sensitivity and
subpicosecond temporal resolution. The observed subsurface density dynamics
serve to validate advanced simulation models depicting matter under extreme
conditions. Our findings promise to unveil novel avenues for laser material
nanoprocessing and high-energy-density science.
