  The structural nature of high-density amorphous ice (HDA), which forms
through low-temperature pressure-induced amorphization of the 'ordinary' ice I,
is heavily debated. Clarifying this question is not only important for
understanding the complex condensed states of H$_2$O but also in the wider
context of pressure-induced amorphization processes, which are encountered
across the entire materials spectrum. We first show that ammonium fluoride
(NH$_4$F), which has a similar hydrogen-bonded network to ice I, also undergoes
a pressure collapse upon compression at 77 K. However, the product material is
not amorphous but NH$_4$F II, a high-pressure phase isostructural with ice IV.
This collapse can be rationalized in terms of a highly effective mechanism. In
the case of ice I, the orientational disorder of the water molecules leads to a
deviation from this mechanism and we therefore classify HDA as a 'derailed'
state along the ice I to ice IV pathway.
