KM3NeT research infrastructure comprises two neutrino telescopes at the bottom of the Mediterranean Sea. The KM3NeT/ARCA telescope is under construction at a depth of ∼3.5 km off-shore Sicily, Italy, and is optimised for the detection of astrophysical neutrinos in the TeV–PeV energy range. The KM3NeT/ORCA detector is being built near the coast of Toulon, France, at a depth of ∼2.5 km. The main physics goals of KM3NeT/ORCA are the determination of neutrino mass ordering and the measurement of oscillation parameters. The overwhelming majority of downward-going events detected by both telescopes are high-energy atmospheric muons. These muons provide unique information for cosmic ray studies and can be explored for the calibration of the KM3NeT detectors. Both topics are covered in this seminar. The first part is devoted to the measurement of the atmospheric muon rate with KM3NeT in comparison with state-of-the-art simulation models. The results are discussed in connection to the ”muon puzzle”, the lack of muons in the simulations with respect to the data collected by extensive air-shower detectors. In the second part, the approach to constrain one of the main systematic uncertainties in the KM3NeT experiment, seawater optical properties, using atmospheric muons is presented.