  Young massive star clusters (YMCs, with M $\geq$10$^4$ M$_{\odot}$) are
proposed modern-day analogues of the globular clusters (GCs) that were products
of extreme star formation in the early universe. The exact conditions and
mechanisms under which YMCs form remain unknown -- a fact further complicated
by the extreme radiation fields produced by their numerous massive young stars.
Here we show that GC-sized clusters are naturally produced in
radiation-hydrodynamic simulations of isolated 10$^7$ M$_{\odot}$ Giant
Molecular Clouds (GMCs) with properties typical of the local universe, even
under the influence of radiative feedback. In all cases, these massive clusters
grow to GC-level masses within 5 Myr via a roughly equal combination of
filamentary gas accretion and mergers with several less massive clusters.
Lowering the heavy-element abundance of the GMC by a factor of 10 reduces the
opacity of the gas to radiation and better represents the high-redshift
formation conditions of GCs. This results in higher gas accretion leading to a
mass increase of the largest cluster by a factor of ~4. When combined with
simulations of less massive GMCs (10$^{4-6}$ M$_{\odot}$), a clear relation
emerges between the maximum YMC mass and the mass of the host GMC. Our results
demonstrate that YMCs, and potentially GCs, are a simple extension of local
cluster formation to more massive clouds and do not require suggested exotic
formation scenarios.
