Very little is known about the hidden light that emits from a select few of Ireland's underwater marine inshore flora and fauna. The phenomenon occurs when certain marine plants and animals that contain fluorescent proteins absorb, transfer and reemit light in a colourful display known as marine fluorescence. Quite a lot of research has been carried out in this area particularly in the tropics amongst coral reefs and the communities of fish that live in and around these habitats. In 2015 I had already photographed bioluminescence (the biochemical emission of light by plankton) at Derrynane and my curiosity led me to believe that other underwater marine animals must emit light too. Then during one dark September night while carrying a 'blue' light on a night dive I noticed that as I directed the beam of my torch on cup corals that they glowed back. My curiosity was awakened. I carried out some research and found that the majority of the literature was from the aforementioned tropical seas. There was nothing relating to temperate marine oceans. Like an excited child I could not wait to investigate and before long I got my hands on a set of blue lights and yellow barrier filters so that I could study and photograph marine fluorescence in more detail. I did not know what to expect nor did I ever imagine that what I was about to witness was simply incredible.