Ly-α tomography surveys have begun to produce 3D maps of the intergalactic medium (IGM) opacity at z∼2.5 with Mpc resolution. These surveys provide an exciting new window to discover and characterize high-redshift overdensities, including the progenitors of today's massive groups and clusters of galaxies. In this talk, I will introduce a novel method for delineating the boundaries of structures detected in 3D Ly-α absorption maps using classical image segmentation techniques. Moreover, I will argue how the Ly-α tomography is complementary to the conventional galaxy surveys by introducing a population of observed protoclusters in the Lyman-α Tomographic IMACS Survey (LATIS), which contains surprisingly few UV-selected galaxies compared with the dark matter halos in mock-observed simulations. Finally, I will discuss the other potential applications of the tomography surveys, such as the synergies with the upcoming line intensity map (LIM) experiments at cosmic noon.