Tunnel ionization in intense infrared laser fields initiates a broad range of processes that evolve on an attosecond time scale. Theoretically, the motion of a liberated electron driven by a strong laser field is described by quantum orbits. In our experiment, the time at which each quantum orbit emerges from under the tunneling barrier is measured and linked to the time when this orbit is brought back to the parent ion by the laser field. Our measurement provides a direct access into the basic properties of tunnel ionization - one of the most fundamental quantum mechanical phenomena - allowing one to independently follow the beginning and the end of each quantum orbit. It calibrates the clock in attosecond spectroscopies, in which tunnel ionization acts as a pump that initiates the dynamics and the return of the electron to the core is used as a probe.