Using recently developed full-dimensional coupled quasi-diabatic ab initio potential energy surfaces including four electronic ((1)pi pi, (1)pi pi*, 1(1)pi sigma* and 2(1)pi sigma*) states, the tunneling dynamics of phenol photodissociation via its first excited singlet state (S-1 <- S-0) is investigated quantum mechanically using a three-dimensional model. The lifetimes of several low-lying vibrational states are examined and compared with experiment. The deuteration of the phenoxyl hydrogen is found to dramatically increase the lifetime, attesting to the tunneling nature of the nonadiabatic dissociation. Importantly, it is shown that owing to the conical intersection topography tunneling in this system cannot be described in the standard adiabatic approximation, which eschews the geometric phase effect since the nonadiabatically computed lifetimes, validated by comparison with experiment, differ significantly from those obtained in that limit.