Gas surface density at the formation site of the most massive (7 × 10⁶ M_⊙) cluster formed in the history of the simulated galaxy, in a 3 × 10⁷ M_⊙ cloud with mean surface density |$\Sigma _{\rm GMC} \sim 1200\, \mathrm{ M}_\odot \rm pc^{-2}$| at z ∼ 0.8. The cloud is found at the edge of a large bubble or cavity, and the galaxy still has a highly irregular morphology. 3D animations of this cloud and the two next-most-massive cluster progenitor clouds can be viewed here.