  The largest galaxies in the Universe reside in galaxy clusters. Using
sensitive observations of carbon-monoxide, we show that the Spiderweb Galaxy -a
massive galaxy in a distant protocluster- is forming from a large reservoir of
molecular gas. Most of this molecular gas lies between the protocluster
galaxies and has low velocity dispersion, indicating that it is part of an
enriched inter-galactic medium. This may constitute the reservoir of gas that
fuels the widespread star formation seen in earlier ultraviolet observations of
the Spiderweb Galaxy. Our results support the notion that giant galaxies in
clusters formed from extended regions of recycled gas at high redshift.
