The impact scientists worry about most in a major spill is more subtle than the presence of oil on feathers or fur. It is the long-term exposure to dispersants and oil that have the potential to trigger dramatic die-offs and population declines. Scientists are not yet able to address these large-scale effects, and will require new methods to monitor and assess the impact of deep, massive leaks far offshore, like the one that resulted from the explosion of the Deepwater Horizon oil rig.