  The goal of the COMPASS experiment at CERN is to study the structure and
spectroscopy of hadrons. The two-stage spectrometer has large acceptance and
covers a wide kinematic range for charged as well as neutral particles allowing
to access a wide range of reactions. Light mesons are studied with negative
(mostly $\pi^-$) and positive ($p$, $\pi^+$) hadron beams with a momentum of
$190\,\text{GeV}/c$.
  The light-meson spectrum is measured in different final states produced in
diffractive dissociation reactions with squared four-momentum transfer $t$ to
the target between $0.1$ and $1.0\,(\text{GeV}/c)^2$. The flagship channel is
the $\pi^-\pi^+\pi^-$ final state, for which COMPASS has recorded the currently
world's largest data sample. These data not only allow us to measure the
properties of known resonances with high precision, but also to search for new
states. Among these is a new axial-vector signal, the $a_1(1420)$, with unusual
properties. The findings are confirmed by the analysis of the $\pi^-\pi^0\pi^0$
final state.
