Undersea eruptions of hydrogen sulphide are killing billions of fish in one of the world's richest fisheries off Africa.
Satellite images show that toxic eruptions off the coast of Namibia are more frequent and widespread than anyone realised.
The fishery was worth around $400 million in 1998, providing Namibia with its second largest source of revenue after mining.
Such productive fisheries are found in upwelling regions of ocean, where wind-driven currents fertilise surface waters with nutrients from the deep.
However, microscopic algae thrive in areas where such upwellings are most intense. Those that are not grazed on by plankton will sink when they die, forming beds of sediment .
Bacteria break this down to produce hydrogen sulphide, which in turn builds up in gas pockets that eventually erupt into the ocean, poisoning marine life and stripping oxygen from the water.
In 1993, oxygen depletion killed two billion young hake - half the fishery's young fish.