DarCo is a Biodiversa+ project (2021–2022 joint call “Supporting the protection of biodiversity and ecosystems”) entitled: "The vertical dimension of conservation: a cost-effective plan to incorporate subterranean ecosystems in post-2020 biodiversity and climate change agendas" (read more). Subterranean ecosystems host a broad diversity of specialized and endemic organisms that account for a unique fraction of the global taxonomic, phylogenetic, and functional diversity. Furthermore, they deliver crucial ecosystem services. Yet, these out-of-sight ecosystems are systematically overlooked in post-2020 biodiversity and climate change targets.Two main impediments are responsible for this lack of protection. First, subterranean biodiversity patterns remain largely unmapped. Second, we lack a mechanistic understanding of subterranean species' response to human-induced perturbations. The project DarCo aims to map subterranean biodiversity patterns across Europe and develop an explicit plan to incorporate subterranean ecosystems in the European Union (EU) Biodiversity Strategy for 2030. DarCo will gather distribution data, traits, and phylogenies for major subterranean animal groups. These data will serve to predict species responses to human threats and design a plan to protect subterranean biodiversity complementing the current EU network of protected areas (Natura 2000). This Topical Issue, curated by Stefano Mammola and Fabio Stoch, and involving Ana Sofia Reboleira, Melissa Meierhofer, and Tiziana di Lorenzo as guest editors, is designed to collect papers produced by DarCo project collaborators and reserches who wish to enrich the knowledge of European subterranean fauna and contribute to its conservation.