When intense Infrared (IR) laser fields interact with atoms or molecules an electron may be liberated through Tunneling Ionization. The tunneled electron is then accelerated by the oscillating field which leads to a broad range of strong field processes. In this work we used one such process, namely High Harmonic Generation (HHG), to explore the electron dynamics under the barrier. By using a combination of strong (IR) and weak (second harmonic) driving fields, we modulate the atomic potential barrier on an optical subcycle time-scale. This induces a temporal interferometer between attosecond bursts originating from consecutive laser half-cycles. Exploiting such an interferometer reveals how some of the basic properties of the tunneling mechanism, such as the tunneling probability, evolve within the ~200 attosecond ionization window. In addition our results suggest a deviation from the “Adiabatic” model of strong field ionization.