  We discover what is in projection the largest known structure of galactic
origin: a giant radio galaxy with a projected proper length of $4.99 \pm 0.04\
\mathrm{Mpc}$. The source, named Alcyoneus, was first identified in
low-resolution LOFAR Two-metre Sky Survey images from which angularly compact
sources had been removed. Being an extreme example in its class, Alcyoneus
could shed light on the main mechanisms that drive radio galaxy growth. We find
that - beyond geometry - Alcyoneus and its host galaxy appear suspiciously
ordinary: the total low-frequency luminosity density, stellar mass and
supermassive black hole mass are all lower than, though similar to, those of
the medial giant radio galaxy (percentiles $45 \pm 3\%$, $25 \pm 9 \%$ and $23
\pm 11 \%$, respectively). The source resides in a filament of the Cosmic Web,
with which it might have significant thermodynamic interaction. At $5 \cdot
10^{-16}\ \mathrm{Pa}$, the pressures in the lobes are the lowest hitherto
found, and Alcyoneus therefore represents one of the most promising radio
galaxies yet to probe the warm-hot intergalactic medium.
