I will revisit the well-known construction of 5d SCFTs from M-theory on a CY3 singularity. Upon massive deformation, such 5d SCFTs are often expected to have 5d N=1 supersymmetric gauge theory descriptions at low energy. I will present a new way to study these 5d ``gauge theory phases'' systematically using type-IIA string theory, and I will comment on the phenomenon of "UV duality." Along the way, I will discuss some slightly subtle properties of the 5d N=1 Coulomb branch prepotential.