When the ocean surface is wavy, surges pass into and out of the cave, causing the water surface here to rise and fall. In extreme conditions, the surface will literally explode out of the opening at times. The ocean was moderately wavy on this day, giving us an active but safe show.
To watch the video, click on the triangle. If your net is not extremely fast, wait until the video starts to play and then click on the pause button (which will appear at the lower left corner of the video window). Allow it to buffer for a while, then click the play button to watch the water dance.