We investigate single Fe and Co atoms buried below a Cu(100) surface using low temperature scanning tunneling spectroscopy. By mapping the local density of states of the itinerant electrons at the surface, the Kondo resonance near the Fermi energy is analyzed. Probing bulk impurities in this well-defined scattering geometry allows separating the physics of the Kondo system and the measuring process. The line shape of the Kondo signature shows an oscillatory behavior as a function of depth of the impurity as well as a function of lateral distance. The oscillation period along the different directions reveals that the spectral function of the itinerant electrons is anisotropic.PACS numbers: 68.37. Ef,72.10.Fk,72.15.Qm, For a long time it has been well known that a localized spin degree of freedom-a magnetic impurity-in a nonmagnetic host metal significantly alters the scattering behavior of the conduction band electrons of the host as compared to nonmagnetic impurities. This results in a variety of thermodynamic anomalies, which are summarized by the term Kondo effect [1]. The most prominent macroscopic hallmark is the resistance minimum at low temperatures observed for metals with magnetic impurities. From a microscopic point of view the impurity interacts with the surrounding electron gas-the itinerant electrons. Below a characteristic temperature, the Kondo temperature T k , a narrow many-body resonance named Kondo or Abrikosov-Suhl resonance builds up in the spectral function of the impurity at the chemical potential and the impurity spin is effectively screened. Electric transport is dominated by electrons near the Fermi energy for low temperature. Hence the Kondo resonance leads to a strong scattering of electrons at the impurity and an increase of resistivity with decreasing temperature.Microscopic properties of single impurity Kondo systems on the atomic scale regained interest by recent scanning tunneling microscopy (STM) experiments [2-7] on single magnetic ad-atoms and molecules deposited on noble metal surfaces (for a review see [8]). In these works the Kondo effect manifests itself as a sharp signature in the STM differential conductance around zero bias. Studies which investigate the dependence of the Kondo signature on the lateral distance [2,5] showed that the signal rapidly vanishes and the line shape nearly remains constant. This is in contrast to theory [9-11] which predicts a long range visibility and oscillatory behavior of the spectral function of the itinerant electrons -the local density of states (LDOS). Models for tunneling through ad-atom systems have to treat the tip, its coupling to the localized impurity state, its coupling to the itinerant bulk electrons as well as surface states, and all interferences between alternative transport paths [10,11].During recent years STM was refined to investigate nano structures [12][13][14][15][16] or even single impurity atoms [17][18][19][20] which are buried below a metal surface. The interference pattern on the surface caused by the sub surface s...