  Tunneling plays a central role in the interaction of matter with intense
laser pulses, and also in time-resolved measurements on the attosecond
timescale. A strong laser field influences the binding potential of an electron
in an atom so strongly, that a potential barrier is created which enables the
electron to be liberated through tunneling. An important aspect of the
tunneling is the geometry of the tunneling current flow. Here we provide
experimental access to the tunneling geometry and provide a full understanding
of the laser induced tunnel process in space and time. We perform laser tunnel
ionization experiments using the attoclock technique, and present a correct
tunneling geometry for helium and argon. In addition for argon the potential
barrier is significantly modified by all the electrons remaining in the ion,
and furthermore from a quantum state whose energy is Stark-shifted by the
external field. The resulting modified potential geometry influences the
dynamics of the liberated electron, changes important physical parameters and
affects the interpretation of attosecond measurements. These effects will
become even more pronounced for molecules and surfaces, which are more
polarizable.
