An orange buoy marks a line leading down to the deep Hydrobox – a large metal structure some 5 metres tall. There’s a round entrance on top of the box, designed to provide a dry working environment for welding and other tasks. If the lid is closed, the box can be filled with air, allowing divers to enter through the gap between the floor and the sides. Look out for the massive ballast weights that keep the box firmly on the bottom of Stoney Cove.