It’s no surprise that coral reefs are in danger. As human activity adds more carbon dioxide to the atmosphere, some of it dissolves in the ocean and leaves the water more acidic and less hospitable. Scientists aren’t as sure how natural submarine groundwater discharge—the underground flow from land out to sea—is affecting coral reefs and the coastal ecosystems they support. Bayani Cardenas from the University of Texas at Austin and a few colleagues suspected that the coast of the reef-rich Calumpan Peninsula that juts off the Philippine island of Luzon would have submarine groundwater discharge, so they collected seawater samples there for three years. The groundwater-fed hydrothermal springs that they discovered, pictured here, belch out so much CO2 that the local concentration is as high as 95 000 ppm; the highest previously reported level, found in Italy in 2008, was 60 000 ppm.
The researchers followed the flow of...
