  Material failure is ubiquitous, with implications from geology to everyday
life and material science. It often involves sudden, unpredictable events, with
little or no macroscopically detectable precursors. A deeper understanding of
the microscopic mechanisms eventually leading to failure is clearly required,
but experiments remain scarce. Here, we show that the microscopic dynamics of a
colloidal gel, a model network-forming system, exhibit dramatic changes that
precede its macroscopic failure by thousands of seconds. Using an original
setup coupling light scattering and rheology, we simultaneously measure the
macroscopic deformation and the microscopic dynamics of the gel, while applying
a constant shear stress. We show that the network failure is preceded by
qualitative and quantitative changes of the dynamics, from reversible particle
displacements to a burst of irreversible plastic rearrangements.
