Groundwater below the island of Spiekeroog flows continuously towards the sea and discharged in the intertidal zone. Since there are no rivers on Spiekeroog, nutrient input to the sea occurs via a subterranean estuary. The aim of the interdisciplinary project BIME was to gain a better understanding of the subterranean estuaries at the North beach on Spiekeroog. In particular, a central component of BIME was the investigation of the spatial and temporal variability of the exchange processes between groundwater and seawater. These complex relationships were investigated by BIME as an interdisciplinary team of chemists, biologists, hydrologists and engineers in close cooperation with the water associations and the national park administration.