Vasilis Gerovasileiou, Assistant Professor at the Department of Environment at the Ionian University and Research collaborator at the Hellenic Centre for Marine Research, the leading partner in LifeWatch Greece, together with Markos Digenis, PhD student also at the Ionian University and HCMR,, describe their work in the well-hidden ecosystems  of 'Marine Caves in the Mediterranean',  Season 3, Episode 11 of our podcast series, 'A Window on Science'. Marine caves in the Mediterranean are protected by the European Union’s Habitats Directive and the Barcelona Convention under the Dark Habitats Action Plan, yet surprisingly little is known about them, compared with other marine habitats. More than 3,000 marine caves have been recorded, mostly on rocky northern Mediterranean coasts. Each one has a unique morphology, and they are all acknowledged as ‘biodiversity reservoirs’ and ‘refuge habitats’ of great conservation value, because they harbour a rich biodiversity, including high numbers of rare, cave exclusive, protected, and deep-sea species.  For the first time, Gerovasileiou & Digenis' research records how sessile communities differ in their structure from the entrance zone of these caves (with plenty of light), to the semi-dark and the dark zone, and analyses the motile fauna of these marine caves with a standardised methodology to understand their role in the ecosystem. Unfortunately, these natural laboratories are being affected by climate change and Invasive Alien Species, although lack of historic data series hinder the evaluation of potential impacts. It is likely that water temperature rise - more intense in the Eastern Mediterranean Sea, closer to the Suez Canal - may facilitate the establishment of non-indigenous species in new areas.