  High-sensitivity $^{27}$Al nuclear magnetic resonance (NMR) measurements of
aluminum metal under hydrostatic pressure of up to 10.1 GPa reveal an
unexpected negative curvature in the pressure-dependence of the electronic
density of states measured through shift and relaxation, which violates free
electron behavior. A careful analysis of the Fermiology of aluminum shows that
pressure induces an electronic topological transition (Lifshitz transition)
that is responsible for the measured change in the density of states. The
experiments also reveal a sudden increase in the NMR linewidth above 4.2 GPa
from quadrupole interaction, which is not in agreement with the metal's cubic
symmetry.
