Trapping light with noble metal nanostructures overcomes the diffraction limit and can confine light to the nanometre scale (typically 30 nm3). Here we demonstrate individual atomic features inside the gap of a plasmonic nano-assembly localise light to volumes well below 1 nm3 (‘picocavities’), enabling optical experiments on the atomic scale. These atomic features are dynamically formed and disassembled by laser irradiation. While unstable at room temperature, picocavities can be stabilised at cryogenic temperatures, allowing single atomic cavities to be probed for many minutes. Unlike traditional optomechanical resonators, such extreme optical confinement yields million-fold-enhanced optomechanical coupling between the picocavity field and vibrations of individual molecular bonds. Picocavity formation probes the nanoscale, accessing sub-molecular dynamics and opens prospects for high precision sensors and single-photon strong-optomechanical coupling.