In this talk, I will present our survey of the most massive overdensities (progenitors of Coma cluster) in the peak of cosmic star formation and QSO activities (z=2--4).  I will present our discovery of a sample of extremely massive overdensies at z~2.3 by utilizing Subaru/HSC, Keck, CFHT and ALMA. The galaxy properties in these overdensities reflect the emergence of the environmental dependence of galaxy property. I will further discuss the discovery of enormous Lyman alpha nebulae in these extremely overdense fields at high redshift. These site are unique laboratories to study the preliminary intracluster medium properties and the IGM-galaxy interactions. Moreover, I will present our new observations of Lyman alpha nebulae using Keck Cosmic Web Imager (KCWI). In the end, I will talk about the future prospects of how greatly improve our understanding of the structure formation in the next decade with LSST, CMB-S4 and spectroscopic telescope.