  A measurement of the four-lepton invariant mass spectrum is made with the
ATLAS detector, using an integrated luminosity of 36.1 fb$^{-1}$ of
proton-proton collisions at $\sqrt{s}$ = 13 TeV delivered by the Large Hadron
Collider. The differential cross-section is measured for events containing two
same-flavour opposite-sign lepton pairs. It exhibits a rich structure, with
different mass regions dominated in the Standard Model by single $Z$ boson
production, Higgs boson production, and $Z$ boson pair production, and
non-negligible interference effects at high invariant masses. The measurement
is compared with state-of-the-art Standard Model calculations, which are found
to be consistent with the data. These calculations are used to interpret the
data in terms of $gg\rightarrow ZZ \rightarrow 4\ell$ and $Z \rightarrow 4\ell$
subprocesses, and to place constraints on a possible contribution from physics
beyond the Standard Model.
