Public perceptions of subterranean renewable energy technologies are shaped not only by technical and environmental considerations but also by deep-seated psychological and cultural associations with the underground. We posit the concept of the “uncanny underground” to describe how interpretations of the subsurface blur boundaries between the familiar and the strange, evoking both curiosity and unease. To explore this phenomenon, we analyze free associations people make with the deep underground, using data collected from a cross-national survey on deep geothermal systems with participants from the US (n = 1003) and Switzerland (n = 1028). While approximately half of the associations describe the physical properties of the underground (e.g., heat, depth, geological structures), a substantial portion reflect symbolic and emotional themes often in the form of dualisms, including: life and death, safety and threat, past and future, and the known and unknown. These associations frequently draw from cultural narratives and symbolic meanings such as burial and the underworld, subterranean adventure stories (e.g., Jules Verne's Journey to the Center of the Earth), and disaster imagery (e.g., The Core). Our findings support research showing that public attitudes towards deep geothermal and other subsurface technologies are shaped by not only technical or economic considerations but also unconscious, culturally embedded meanings that a lifetime of experience and learning create. Understanding how these meanings influence people's perceptions of what we call the “uncanny underground” provides valuable insight into how individuals might react to subterranean energy technologies aimed at addressing climate change.