Snapshots of a GC breaking and forming event observed in our simulations are shown in Figure and. We now address the question of how to test our GC hypothesis. It is a challenging task to unambiguously and automatically identify GCs. We adopt the following approach. If a proton begins its diffusion path at a given O atom, traverses at least 2 other O atoms and then returns to the starting O atom we label this a "traceback" event. A traceback of length N (i.e., containing N O atoms) is sufficient (but not necessary) proof of the existence of a GC of length N . We have measured the traceback time, which is the time required for a CEC to traverse the length of the GC and return to the original O atom. We use the traceback time as a surrogate for the GC lifetime in our analysis, recognizing that a less restrictive definition of a GC (not requiring a traceback event) may have significantly shorter lifetimes. A plot of the histogram of traceback times for N = 3, 4, 5, 6 is given in Figure . The traceback times follow a log-normal distribution. The log transform of the data (which is Gaussian) is shown in Figure , where we also show the histograms for individual GC lengths. The mean traceback time was calculated to be 240 fs. This does not mean that the average lifetime of a GC is close to this value. It means that some of the GCs have lifetimes long enough to enable traceback events, and these events take, on average, about 240 fs. Nevertheless, it does prove the existence of GCs and that they can have lifetimes that are relatively long compared to proton hopping events over multiple OH groups.