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We use a combination of X-ray diffraction, total scattering and quantummechanical calculations to determine the mechanism responsible forhydration-driven contraction in ZrW$_2$O$_8$. Inclusion of H$_2$O moleculeswithin the ZrW$_2$O$_8$ network drives the concerted formation of new W--Obonds to give one-dimensional (--W--O--)$_n$ strings. The topology of theZrW$_2$O$_8$ network is such that there is no unique choice for the stringtrajectories: the same local changes in coordination can propagate with a largenumber of different periodicities. Consequently, ZrW$_2$O$_8$ is heavilydisordered, with each configuration of strings forming a dense aperiodic`spaghetti'. This new connectivity contracts the unit cell \emph{via} largeshifts in the Zr and W atom positions. Fluctuations of the undistorted parentstructure towards this spaghetti phase emerge as the key NTE phonon modes inZrW$_2$O$_8$ itself. The large relative density of NTE phonon modes inZrW$_2$O$_8$ actually reflect the degeneracy of volume-contracting spaghettiexcitations, itself a function of the particular topology of this remarkablematerial.

Negative Hydration Expansion in ZrW2O8: Microscopic Mechanism, Spaghetti Dynamics, and Negative Thermal Expansion