2025ApJ...993L..49S

Protoclusters, galaxy clusters' high-redshift progenitors, hold the key to understanding the formation and evolution of clusters and their member galaxies. However, their cosmological distances and spatial extensions (tens of Mpc) have inhibited complete mapping of their structure and constituent galaxies, which is key to robustly linking protoclusters to their descendants. Here we report the discovery of the Bigfoot, a tridimensional structure at z = 3.98 including 11 subgroups traced by 55 (700) spectroscopic (photometric) redshifts with JWST, extending over 15 × 37 × 49 cMpc³ in the PRIMER-UDS field. Bigfoot's large-scale and mass function of member galaxies closely match constrained simulations' predictions for the progenitors of today's most massive clusters (M₀ > 10¹⁵M_⊙). All subgroups with M_h > 10^12.5M_⊙ exhibit enhanced fractions of massive galaxies (>10^10.0M_⊙) compared to lower-mass halos and fields, demonstrating the accelerated formation of massive galaxies in massive halos. The presence of this massive protocluster with a large central halo (10^13.0M_⊙) in a JWST deep field bears important cosmological implication that favors high σ₈ of PLANCK cosmology over low-redshift probes.