This photograph, taken from the International Space Station (ISS), shows the north coast of Trinidad and a series of subtle, interacting arcs in the southeastern Caribbean Sea. These are known as "internal waves," the surface manifestation of slow waves that move tens of meters beneath the sea surface. Internal waves produce enough of an effect on the sea surface to be seen from space, but only where they are enhanced due to reflection of sunlight, or sunglint, back towards the International Space Station. The image shows at least three sets of internal waves interacting. The most prominent set (image top left) shows a packet of several waves moving from the northwest due to the tidal flow towards the north coast of Trinidad. Two less prominent, younger sets can be seen further out to sea. A very broad set enters the view from the north and northeast, and interacts at image top center with the first set. All the internal waves are probably caused by the shelf break near Tobago (outside the image to top right). The shelf break is the step between shallow seas (around continents and islands) and the deep ocean. It is the line at which tides usually start to generate internal waves.