At this granite ledge, the best time to witness the boom and spray is two hours before high tide. If the ocean is calm or the tide is low, only gurgling may be heard. Over eons, the restless forces of the ocean gouged a narrow cave in the ledge, and when it suddenly fills with water, trapped air creates a thunderous burst of ocean spray. Formations like Thunder Hole show the sheer power of coastal waves and currents. Big waves caused by high tides or offshore storms can occur without warning and have washed people out to sea, and black algae and water make shore rocks very slippery.