The sea around Koh Rong lights up when you touch it, and not for your benefit. Cambodia's second-largest island has forty-three of its sixty-one kilometres of coast in bare beach, and on a moonless night the shallows answer every movement with blue-green sparks. The light comes from single-celled plankton, and it is a defence. Biologists have argued for years about how it works: the flash may startle the grazers eating the plankton, or warn them off as a signal that the cell is toxic, or do something colder. The explanation most accept is a burglar alarm.
In daylight the island is sand and jungle. The water has been a national marine park since 2018, Cambodia's first, patrolled against trawlers, and the coral gardens off the bays are shallow enough to snorkel from the sand. In the dry months the Gulf flattens and you can read the ripples on the seabed from the boat. A trail crosses the jungle spine from Koh Touch in under an hour, macaques overhead and monitor lizards sliding off the path ahead of you. It comes out at Sok San: kilometres of beach with almost nothing built on it. Other strips have no road at all and can only be reached by boat. Most families here still live off fishing, and the village piers run far out over clear shallow water, busiest early and late when the boats unload and the nets are mended. The glow is strongest late in the wet season, which is also when the sea is roughest. Away from the village lights, a hand pushed through black water fires a spray of pinpricks that fade in a second, and your wake glows behind you. As a burglar alarm it works like this: the cell flashes, the grazer that seized it bolts, and the jump is what shows a larger, hungrier animal where it is.