MAKING WAVES: Sounds of the underground - unveiling the potential of acoustic monitoring in groundwater ecosystems.

Groundwater ecosystems play a key role in maintaining the ecological integrity of the global water cycle, yet they seldom take centre stage in biodiversity conservation agendas. Within such cryptic environments, this bottleneck is partly triggered by their inaccessibility but also the lack of combination between - often time-consuming and expensive - conventional hydro-ecological surveys and novel biomonitoring tools. Acoustic approaches, widely implemented in other aquatic environments such as oceans and inland waters, have the potential to ease current roadblocks and hit the right note in terms of increased groundwater protection. Here, we briefly review current use (and derived insights) of acoustic tools in groundwaters and also leverage current technical and analytical developments from other disciplines to advance groundwater monitoring. By targeting the three major branches of acoustic research, we discuss the potential of using sounds to decipher the environmental conditions in groundwaters (hydroacoustics), to unravel the biotic trends across subterranean animals (bioacoustics), and to investigate the dynamic at community level (ecoacoustics). With the goal of banging the drum for increased acoustic research in groundwaters, we conclude by reflecting on four acoustic applications that could significantly refine our understanding of natural and anthropogenically derived conditions underground. When integrated within groundwater routine surveys, acoustics not only will consistently improve the way we assess the groundwater ecosystem but will also allow to turn up the volume on one of the most fundamental but threatened ecosystems on the planet.