The hydrogen Lyman-alpha forest is a long-established probe of large-scale structure at z>2, but is typically limited to 1D investigations along individual quasar sightlines. However, by instead targeting LBGs as background sources, the transverse separation between sources is ~Mpc and it becomes possible to do a 3D 'tomographic' reconstruction of the intergalactic medium. I will describe pilot observations using this technique, which has produced the first map of 3D large-scale structure at z~2.3 within the COSMOS field. Comparisons with coeval galaxies and simulations indicate that our map is truly tracing large-scale structure. This motivates the CLAMATO survey, which will map out a volume of (100 Mpc/h)^3 at z~2.3 and allow us to search for galaxy protoclusters, study the effect of environment on galaxy evolution, and constrain cosmological parameters.