A steady increase of on-target laser intensity with also increasing pulse contrast is leading to light-matter interactions of extreme laser fields with matter in new physics regimes. At the Texas Petawatt laser we have realized interactions in the transparent-overdense regime, which is reached by interacting a highly relativistic, ultra-high contrast laser pulse with a solid density ultrathin target. The extreme fields in the laser focus are turning the overdense, opaque target transparent to the laser by the relativistic mass increase of the electrons. Thus, the interaction becomes volumetric, increasing the energy coupling from laser to plasma. Using plasma mirrors to increase the on-target contrast ratio, we demonstrated generation of over 60 MeV proton beams with pulse energies not exceeding 40 J (on target).