Rip currents can form by the coasts of oceans, seas, and large lakes, whenever there are waves of sufficient energy. Rip currents often occur on a gradually shelving shore, where breaking waves approach the shore parallel to it, or where underwater topography encourages outflow at one specific area. The location of rip currents can be difficult to predict; some tend to recur always in the same places, but others can appear and disappear suddenly at various locations along the beach. The appearance and disappearance of rip currents is dependent on the bottom topography and the exact direction from which the surf and swells are coming in.