Marine life caught the brunt of volcanism 93 million years ago. While dinosaurs still roamed the land, a surge of undersea eruptions wiped out many of the ocean's species, according to a study last year.
The Earth was already much warmer than it is today, and the sea was thick and sluggish, the study authors say. And when shifting tectonic plates set off a spike of undersea eruptions, the resulting clouds of ash choked off the ocean's oxygen supply. The algae, clams, and other sea creatures that were killed sank to the sea floor and became some of the oil deposits that modern drillers extract today.