  Crystals spontaneously break the continuous translation symmetry in space,
despite the invariance of the underlying energy function. This has triggered
suggestions of time crystals analogously lifting translational invariance in
time. Originally suggested for closed thermodynamic systems in equilibrium,
no-go theorems prevent the existence of time crystals. Proposals for open
systems out of equilibrium led to the observation of discrete time crystals
subject to external periodic driving to which they respond with a sub-harmonic
response. A continuous time crystal is an autonomous system that develops
periodic auto-oscillations when exposed to a continuous, time-independent
driving, as recently demonstrated for the density in an atomic Bose-Einstein
condensate with a crystal lifetime of a few ms. Here we demonstrate an
ultra-robust continuous time crystal in the nonlinear electron-nuclear spin
system of a tailored semiconductor with a coherence time exceeding hours.
Varying the experimental parameters reveals huge stability ranges of this time
crystal, but allows one also to enter chaotic regimes, where aperiodic behavior
appears corresponding to melting of the crystal. This novel phase of matter
opens the possibility to study systems with nonlinear interactions in an
unprecedented way.
