Mass extinctions that wiped out up to 90% of earth's flora and fauna were driven in large part by shifting ocean levels, according to a study published in Nature. Understanding what made many of the planet's living organism rapidly die out at least five times over the last half billion years remains one of the great challenges in paleontology and biology. Some theories point an accusing figure at the cooling effect of massive dust shrouds thrown into the atmosphere by volcanoes and asteroids crashing into earth, or the warming caused by rising levels of carbon dioxide.