This image shows lava flowing from the edge of a cliff into an ocean. I think the image is fascinating because it shows two very different substances acting in a similar way. Molten rock is thousands of degrees hotter than liquid water and it is sometimes odd to think about a rock in liquid form. Seeing the lava flow over a cliff in a similar way to a waterfall shows that flow patterns are almost universal, even between very different materials.
Photo credit: Bruce Omori