“[Two percent] is pretty significant,” says Sune Nielsen, a geochemist at the Woods Hole Oceanographic Institution in Massachusetts. “That’s actually pretty scary.”

Nielsen is one of a group of scientists probing a series of strange ancient catastrophes when the ocean lost much of its oxygen for insight into our possible future in a suffocating world. He has studied one such biotic crisis in particular that might yet prove drearily relevant. Though little known outside the halls of university labs, it was one of the most severe crises of the past 100 million years. It’s known as Oceanic Anoxic Event 2.

The Mesozoic era, stretching from 252 to 66 million years ago, is sometimes mistakenly thought of as sort of long and uneventful Pax Dinosauria—a stable, if alien world. But the period was occasionally punctuated by severe climate and ocean changes, and even disaster. Ninety-four million years ago, while the supersonic asteroid that would eventually incinerate dinosaurs was still silently boomeranging around the solar system, a gigantic pulse of carbon dioxide rose from the bottom of the ocean. The Earth warmed, the seas rose, and oxygen-deprived waters spread. The smothering seas mercilessly culled through plankton, bizarre bivalves, and squid-like creatures whose tentacles long dangled from stately whorled shells. For the dolphin-like ichthyosaurs, Oceanic Anoxic Event 2, or OAE 2, might have been the coup de grâce. The ocean reptiles had been patrolling the ancient seas for more than 150 million years before seemingly taking their last gasps suspiciously close to the event.

“Basically the entire continental shelf went anoxic,” says Nielsen. “There was no oxygen at the bottom of the shelf anywhere in the world.”

Today, as much as 90 percent of commercial fish and shellfish are caught on these shallow shelves—the broad flanks of our continents that slip coyly under the sea, sometimes for hundreds of miles, before remembering to drop off into the abyss. And already, spreading anoxia is beginning to advertise its deadly promise on these fishing grounds: In 2006 a seafloor survey off of Oregon revealed that rockfish, familiar fixtures of the rocky bottom, had completely abandoned their haunts, as anoxic water—water with no dissolved oxygen—spread onto the shallow shelf. But 94 million years ago in the Cretaceous, this problem was not just a seasonal nuisance. It was a global catastrophe. If dromaeosaurs had learned to pilot industrial bottom trawlers on the continental shelf, they would have gone bankrupt pulling up empty nets.

The source of the great smothering in the Cretaceous seems to have been a molten font burbling deep beneath an ancient sea that separated North from South America. The lava from these eruptions makes up much of what today is known as the Caribbean large igneous province, a vast expanse of frozen lava that stretches from Ecuador in the Pacific to the Antilles bracing against the open Atlantic. Like many scientific sobriquets, “large igneous province” fails utterly to capture the phenomenon it describes—though no description could ever really succeed in evoking its terrible grandeur.