* Rift valleys: These are long, narrow valleys that form along the axis of mid-ocean ridges. They are caused by the spreading of the seafloor and the upwelling of magma from the mantle.

* Hydrothermal vents: These are areas on the seafloor where hot water from the Earth's crust mixes with cold water from the ocean. This mixing of water creates a unique environment that supports a variety of specialized organisms.