  We study closed string axions in type IIB orientifold compactifications. We
show that for natural values of the background fluxes the moduli stabilisation
mechanism of the LARGE Volume Scenario (LVS) gives rise to an axiverse
characterised by the presence of a QCD axion plus many light axion-like
particles whose masses are logarithmically hierarchical. We study the
phenomenological features of the LVS axiverse, deriving the masses of the
axions and their couplings to matter and gauge fields. We also determine when
closed string axions can solve the strong CP problem, and analyse the first
explicit examples of semi-realistic models with stable moduli and a QCD axion
candidate which is not eaten by an anomalous Abelian gauge boson. We discuss
the impact of the choice of inflationary scenario on the LVS axiverse, and
summarise the astrophysical, cosmological and experimental constraints upon it.
Moreover, we show how models can be constructed with additional light
axion-like particles that could explain some intriguing astrophysical
anomalies, and could be searched for in the next generation of axion
helioscopes and light-shining-through-a-wall experiments.
