  The galaxy Malin 1 contains the largest stellar disk known but the formation
mechanism of this structure has been elusive. In this paper, we report a Malin
1 analogue in the 100 Mpc IllustrisTNG simulation and describe its formation
history. At redshift zero, this massive galaxy, having a maximum circular
velocity $V_{\rm max}$ of 430 ${\rm km\ s^{-1}}$, contains a 100 kpc
gas/stellar disk with morphology similar to Malin 1. The simulated galaxy
reproduces well many observed features of Malin 1's vast disk, including its
stellar ages, metallicities, and gas rotation curve. We trace the extended disk
back in time and find that a large fraction of the cold gas at redshift zero
originated from the cooling of hot halo gas, triggered by the merger of a pair
of intruding galaxies. Our finding provides a novel way to form large galaxy
disks as extreme as Malin 1 within the current galaxy formation framework.
