Illustration of seawater flowing through the upper oceanic crust via two seamounts (extinct underwater volcanoes) that act as a siphon. From centre left; cold seawater (blue arrows) penetrates the porous volcanic rock of a large seamount and travels horizontally through the crust. As it does so, it is warmed by heat from the Earths interior. When the water reaches a smaller seamount it is discharged back into the sea. This flow can occur over distances of at least 50km. The amount of water that circulates through the upper oceanic crust is equivalent to the combined flows of all the rivers on Earth.