From the blue, it glides towards me: a huge, dark manta ray, swimming effortlessly from the depths. It reaches the surface and floats motionless like a creature from another world. Then it banks left and swims lower in a series of loops just below me, exposing its white underbelly and 3 metre wing span. Then it dives deep, only to return for another fly-by. Its graceful moves and massive size leave me breathless. Every diver dreams of encounters like this but are they becoming increasingly rare? There may come a day when few - if any - manta rays remain.