The seafloor covers more than half of the Earth's surface. Most of the seafloor consists of the flat, expansive abyssal plain. But the landscape is very different where cracks in the seafloor allow molten rock to well up from below. In these volcanic areas, there are hot-water vents, giant mountain ranges, and deep trenches. Communities of bizarre organisms feast on dead animals that fall from above.

Definition: the vast landscape of the ocean bottom
Characteristics: expansive plains, mountains, trenches, ridges, and hydrothermal vents
Where: in all the oceans of the Earth
Inhabitants: over 200,000 mud-dwelling animals, most of which are tiny one-celled organisms
Significance: one of the most diverse ecosystems on Earth

Image credits: main image, courtesy of NOAA, Ocean Explorer, Alvin.