Some of the best things on this coast are free, and one of them only happens in the dark. On the right night, the sea around Cambodia's islands glows, a cold electric blue that answers every footstep in the shallows and every stroke of a paddle. It sounds like a traveller's tall tale, right up until you see it.
What it is
The glow comes from bioluminescent plankton, tiny organisms that flash a brief blue-green light when the water around them is disturbed. Stir the shallows and they spark; a swimmer trails light, a paddle drips fire, a breaking wave draws a glowing line along the sand. It is the same chemistry that lights a firefly, scaled up to the whole sea.
Where to find it
The islands are the place, especially Koh Rong and Koh Rong Sanloem, where the water is clear and parts of the coast stay genuinely dark. The plankton are there more often than not, but you cannot see a faint glow against a bright background, so the real trick is darkness. Walk away from the bars and the generator lights, to the quiet end of a beach or out on a night boat, and give your eyes ten minutes to adjust.
How to see it at its best
Go on a moonless night, the darker the better, well after the resort lights have dimmed. Avoid the full moon, when the sky washes the glow out. A gentle swim or a slow kayak is the finest way to experience it, every movement answered in light, though even trailing a hand from a boat will do it. Resist the urge to film, since phones rarely catch it and you are better off just watching.
How to be here
Ask your guesthouse when the moon is dark and which beach stays unlit, bring a torch for the walk, and switch it off when you arrive. The sea will do the rest. It is the kind of wonder no one can package, which is precisely why it stays with you.