Although we cannot see it happening, oceans are constantly coming into existence on the planet. This process, which takes tens of millions of years, begins when two continental plates spread apart and magma wells up through the fissures. The ocean crust gets thinner, bulges, and then sinks, forming a rift valley. Water gradually flows into the new valley as the divergence continues. Accumulating lava forms new oceanic crust, while the old crust is pushed outward. Along the fracture zone, the crust folds like a carpet to form new mountains at the bottom of the ocean.