There is some evidence that average wave heights are slowly rising, and that freak waves of eighty or ninety feet are becoming more common. Wave heights off the coast of England have risen an average of 25 percent over the past couple of decades, which converts to a twenty-foot increase in the highest waves over the next half century. One cause may be the tightening of environmental laws, which has reduced the amount of oil flushed into the oceans by oil tankers.