The star-forming, sub-millimeter galaxy (SMG) population represent the most intense stellar nurseries in the Universe. Their high star formation rates of 200-2000 Msun/yr (compared to the Milky Way’s 1 Msun/yr) pose a unique challenge for cosmological simulations of how galaxies form and evolve, particularly in the first few billion years after the Big Bang. Although rare today, these unusual galaxies were factors of 1000 times more prevalent 10 billion years ago, contributing significantly to the buildup of the Universe’s stars during catastrophic galaxy-galaxy collisions that ignited shortlived but extremely powerful bursts of star formation.

Their presence is ubiquitous in massive galaxy “protoclusters” — the precursors of rich galaxy clusters, the most massive gravitationally bound objects in the Universe. They pose intriguing questions regarding the formation of the cluster cores, the assembly of the “Brightest Cluster Galaxies” in the central cluster regions, and generally about the collapse of the cosmic web over large scales. I will a present an ongoing survey with the South Pole Telescope to search for protoclusters of galaxies in the early (z>3) Universe, and discuss what they teach us about galaxy growth and the collapse of large scale structure in an evolving Universe.