One ocean binds together our world. Of that vast expanse of water, less than one-fourth of 1 percent is occupied by coral reefs -- yet reefs are home to more than a quarter of all known marine fish species. Anyone who dives knows that these reefs are dying, but few understand the extent to which human activities threaten these undersea rainforests. The living dead? Photo: NOAA. Four years ago, to help gauge the situation, my colleagues produced the first-ever detailed, map-based assessment of threats to coral reefs around the world. The map revealed a grim truth: 58 percent of the …