Subterranean environments contribute to three-quarters of classified ecosystem services.

Beneath the Earth's surface lies a network of interconnected caves, voids, and systems of fissures forming in rocks of sedimentary, igneous, or metamorphic origin. Although largely inaccessible to humans, this hidden realm supports and regulates services critical to ecological health and human well-being. Subterranean ecosystems are integral to major biogeochemical cycles, sustain diverse surface habitats, and serve as the primary source of irrigation and drinking water. They also offer non-material benefits, including scientific discovery, education, and cultural practices. Yet, these contributions often go unrecognised, partly due to the lack of a unified synthesis of ecosystem services across terrestrial, freshwater, and marine subterranean compartments. This gap limits effective communication of their value to scientists, practitioners, and the public. Through a systematic expert-based review, we show that subterranean ecosystems contribute to up to 75% of classified ecosystem services. Notably, many of these contributions are described only qualitatively, lacking numerical or economic quantification. Next, we list examples of the main ecosystem services provided by subterranean systems to offer a global overview of their multifaceted value and vulnerability to environmental change. We believe this synthesis provides researchers and practitioners with concrete examples to communicate more effectively the importance of subterranean ecosystems to diverse audiences.