For the past 20 years, the Pierre Auger Observatory has collected the largest dataset of ultra-high-
energy cosmic rays (UHECRs) ever achieved using a hybrid detector. The study of this dataset
has led to numerous unexpected discoveries that enhance our understanding of the origins of
UHECRs. One of the key points in this study is their mass composition. In this work, we will
present the most recent results regarding the mass composition of UHECRs at the Pierre Auger
Observatory. In particular, we will focus on the measurement of the depth of the maximum of
air-shower profiles, denoted as Xmax . This determination has been achieved through both direct
measurements from the Fluorescence Detector data and the application of machine learning for
estimating Xmax on an event-by-event basis using the Surface Detector data. The latter has allowed
us to extend the measurement to energies up to 100 EeV and indicates a correlation between changes
in composition and three features of the energy spectrum (ankle, instep, steepening). Moreover,
the results provide evidence of a heavy and nearly pure primary beam for energies greater than
50 EeV that is independent of the hadronic interaction model. The implications of these findings
for astrophysics and for modelling hadronic interactions will be discussed.