Acapulco Mexico is built around the edge of a circular bay rimmed by mountains. The beach is all pebbles rather than sand, and about a mile out is a little island in the center of the bay. Every time we'd walk on the beach, (we'd feel more than hear) there'd be a rumbling and a little vibration you'd feel on the soles of your bare feet. Finally I asked someone and learned....Acapulco Bay is a volcano rim....the island is the cone plug! They say it never stops rumbling...pretty cool