Underwater acoustic and environmental measurements were made adjacent to the Monitor-Merrimac Memorial Bridge-Tunnel in the James River in Virginia during a field experiment in April 2019. The observed sound field exhibited short time-scale and tidal time-scale variability due to passing marine vessels and tidally driven currents in the estuary; most interestingly, the sound field exhibited variability on a diurnal time scale, closely correlated to the temporal distribution of vehicular traffic in the underwater tunnel. During morning and evening rush-hour traffic periods, the amplitude of the traffic-induced underwater signal exceeded 10 dB at 40 Hz. Additional physics-based modeling and direct observation are required to verify the hypothesis that the signal is associated with the traffic-induced vibration in the tunnel roadbed.