  Fabrication of graphene structures has triggered vast research efforts
focused on the properties of two-dimensional systems with massless Dirac
fermions. Nevertheless, further progress in exploring this quantum
electrodynamics system in solid-state laboratories seems to be limited by
insufficient electronic quality of manmade structures and the crucial question
arises whether existing technologies have reached their limits or major
advances are in principle possible. Here we show that graphene in a
significantly purer state can be found in nature on the surface of bulk
graphite, in form of flakes decoupled from the substrate material. Probing such
flakes with Landau level spectroscopy in the THz range at very low magnetic
fields, we demonstrate a superior electronic quality of these ultra low density
layers (n~3x10^9 cm^-2) expressed by the carrier mobility in excess of 10^7
cm^2/(V.s). This finding represents an important challenge for further
improvements of current graphene technologies.
