The KM3NeT Collaboration is constructing neutrino detectors at depths of 2475 m and 3400 m in the Mediterranean Sea, on scales up to a gigaton. These detectors, named ARCA and ORCA are each made up of a three-dimensional array of spherical optical modules. Each of them contains 31 3" photomultiplier tubes, designed to detect Cherenkov light emitted by charged leptons produced by neutrino interactions in and around the instrumented volume. These are packed either sparsely (ARCA) or densely (ORCA), depending on the target energy. ORCA, which is under construction off the coast of Toulon in France, will study atmospheric neutrino oscillations in the 1-100 GeV range. This will address multiple outstanding issues in neutrino oscillation research, including the determination of the neutrino mass hierarchy. Physics studies indicate that this can be determined with a significance of 3-7 sigma (depending on the true value of the hierarchy and the value of the mixing angle θ23) after three years of operation.