Ice is known to collapse to amorphous ice upon pressurization at low temperatures and shows the unusual feature of multiple distinct solid amorphous water states, which have inspired models of liquid water’s structure and unusual properties.Here, we use heat capacity Cp measurements to show that similarly collapsed ice clathrates display identical glass behavior as amorphous ice but that crystallization above the glass transition temperature Tg of ∼140 K at 1 GPa is inhibited. This effect of the homogeneously distributed “guest molecules” in water reveals a relatively strong reversible Cp increase above Tg but no further transition before crystallization at ∼190 K.This is consistent with a glass−liquid transition of water at Tg, which suggests a new path to study an ultraviscous liquid water network and evaluate water models