  Water - tert-butyl alcohol (TBA) binary mixture exhibits a large number of
thermodynamic and dynamic anomalies. These anomalies are observed at
surprisingly low TBA mole fraction, with $x_{\text{TBA}} \approx 0.03 - 0.07$.
We demonstrate here that the origin of the anomalies lies in the local
structural changes that occur due to self-aggregation of TBA molecules. We
observe a percolation transition of the TBA molecules at $x_{\text{TBA}}
\approx 0.05$. We note that "islands" of TBA clusters form even below this mole
fraction, while a large spanning cluster emerges above that mole fraction. At
this percolation threshold, we observe a lambda-type divergence in the
fluctuation of the size of the largest TBA cluster, reminiscent of a critical
point. Alongside, the structure of water is also perturbed, albeit weakly, by
the aggregation of TBA molecules. There is a monotonic decrease in the
tetrahedral order parameter of water, while the dipole moment correlation shows
a weak non-linearity. Interestingly, water molecules themselves exhibit a
reverse percolation transition at higher TBA concentration, $x_{\text{TBA}}
\approx 0.45$, where large spanning water clusters now break-up into small
clusters. This is accompanied by significant divergence of the fluctuations in
the size of largest water cluster. This second transition gives rise to another
set of anomalies around. Both the percolation transitions can be regarded as
manifestations of Janus effect at small molecular level.
