Scientific studies of the environmental effects of oil spills in the United States have produced a steady stream of unexpected discoveries countering prior and often simplistic assumptions. In this brief review, I present how major discoveries from scientific studies of oil spill effects on marine ecosystems and environments, beginning with the 1989 Exxon Valdez, have led to a more informed appreciation for the complexity and the severity of the damage that major spills can do to marine ecosystems and to an increasing recognition that our ability to evaluate those damages is very limited, resulting in a structural bias toward underestimation of adverse environmental effects.