  The Widom line identifies the locus in the phase diagram where a
supercritical gas crosses over from gas-like to a more liquid-like behavior. A
similar transition exists in correlated electron liquids, where the interplay
of Coulomb repulsion, bandwidth and temperature triggers between the Mott
insulating state and an incoherent conduction regime. Here we explore the
electrodynamic response of three organic quantum spin liquids with different
degrees of effective correlation, where the absence of magnetic order enables
unique insight into the nature of the genuine Mott state down to the most
relevant low-temperature region. Combining optical spectroscopy with
pressure-dependent dc transport and theoretical calculations, we succeeded to
construct a phase diagram valid for all Mott insulators on a quantitative
scale. In the vicinity of the low-temperature phase boundary, we discover
metallic fluctuations within the Mott gap, exhibiting enhanced absorption upon
cooling that is not present in antiferromagnetic Mott insulators. Our findings
reveal the phase coexistence region and Pomeranchuk-like anomaly of the Mott
transition, previously predicted but never observed.
