" But the sound frightened Isaac. The thudding, he knew, was caused by great deep-ocean swells falling upon the beach. Most days the Gulf was as placid as a big lake, with surf that did not crash but rather wore itself away on the sand. The first swells had arrived Friday. Now the booming was louder and heavier, each concussion more profound. "

― Erik Larson , Isaac's Storm: A Man, a Time, and the Deadliest Hurricane in History