Coming to a body of water near on you.

Dead zones – areas of oxygen-depleted bottom waters – are spreading at an alarming rate in coastal waters, killing off huge amounts of marine life, a new study has found.

In a paper published today in Science, Robert Diaz, a biological oceanographer at the Virginia Institute of Marine Science, and Rutger Rosenberg, a marine ecologist at Sweden’s Göteborg University, identified more than 400 dead zones worldwide, affecting an area of more than 95,000 square miles, an area roughly the size of Oregon. The number and size of these dead zones are far greater than previously estimated.

While some dead zones occur naturally, many are caused or exacerbated by chemical fertilizer runoff, fossil fuels, and rain. The fertilizer, which is rich in nitrogen compounds, is washed away from farmlands into rivers and ends up in the ocean. Burning fossil fuels produces airborne nitrogen oxides, which the rain washes into the ocean.

The nitrogen compounds feed massive algae blooms. When the algae dies, it sinks to the ocean floor where it is consumed by microbes, which also consume oxygen in the process. As the oxygen is depleted, creating a condition called hypoxia, marine life that can flee does, and life that cannot – some fish but also clams, crustaceans, and other bottom dwellers – die of asphyxiation. At that point, microbes that live in oxygen-free environments begin to thrive and produce hydrogen sulfide, a poisonous gas. Most dead zones are seasonal, as the algae thrives in warm water.