  Metal-insulator transitions in clean, crystalline solids can be driven by two
distinct mechanisms. In a conventional insulator, the charge carrier
concentration vanishes, when an energy gap separates filled and unfilled
electronic states. In the established picture of a Mott insulator, by contrast,
electronic interactions cause coherent charge carriers to slow down and
eventually stop in electronic grid-lock without materially affecting the
carrier concentration itself. This description has so far escaped experimental
verification by quantum oscillation measurements, which directly probe the
velocity distribution of the coherent charge carriers. By extending this
technique to high pressure we were able to examine the evolution of carrier
concentration and velocity in the strongly correlated metallic state of the
clean, crystalline material NiS$_2$, while tuning the system towards the Mott
insulating phase. Our results confirm that pronounced electronic slowing down
indeed governs the approach to the insulating state. However, the critical
point itself, at which the carrier velocity would reach zero and the effective
carrier mass diverge, is concealed by the insulating sector of the phase
diagram. In the resulting, more nuanced view of Mott localisation, the
inaccessibility of the low temperature Mott critical point resembles that of
the threshold of magnetic order in clean metallic systems, where criticality is
almost universally interrupted by first order transitions, tricritical
behaviour or novel emergent phases such as unconventional superconductivity.
