Brittle materials fail via rapidly propagating cracks. Fast cracks are unstable to microscopic branching (micro-branching), making fracture a truly 3D phenomenon. 

We follow in-plane crack front dynamics via real-time measurements in brittle gels as micro-branching unfolds and progresses. We first show that spatially local
energy balance quantitatively describes crack dynamics, even when translational invariance is badly
broken. Furthermore, our results explain micro-branch dynamics; why micro-branches form along
spatially localized chains and how finite-time formation of cusps along the crack front leads to their
death.