A wide variety of devices are based on maintaining high electric fields between metallic electrodes held in vacuum. Examples range from X-ray sources and particle accelerators to vacuum breakers. In most cases, the operational range of the device is limited by the maximal electric field that can be held reliably above the metallic surface. At high electric fields, plasma formation inside the vacuum can lead to breakdown [BD] and failure of the device. Even though field breakdown presently is a widely occurring phenomenon, as of today the mechanism of its formation remains elusive. In this study we explore pre- and post- BD phenomenon in OFHC copper samples that are exposed to high fields within the context of the CERN/CLIC project, which is dedicated to design a future linear accelerator. We report on liquid-like structures appearing on surfaces exposed to RF and DC high electric fields that we observed using SEM, and demonstrate the similarities between the observed structures. We also used DB FIB (Dual Beam Focused Ion Beam) and TEM to study sub-surface plastic damage and the evolution of the dislocation structure due to field exposure. We observe that the plastic zone below the BD site surface is larger than that which is below liquid-like structures. Furthermore, it is demonstrated that dislocation density changes in the samples exposed to electric field. The observed sub-surface plastic activity can serve to constrain models for breakdown nucleation as well as offer new directions for improving metallic electrode performance.