The vast majority of the real estate at the bottom of the sea is a relatively flat expanse of level floor called the abyssal plain. It averages over two miles deep and accounts for as much 79% of the sea floor over the entire planet. The top layer of sediment found on the surface of the abyssal plain is rich in organic matter that has drifted down from above and contains the remains of virtually everything that lives in the upper reaches to the midwater ranges of the ocean. All creatures that spend their lives at sea are also buried at sea, their remains being eaten by other organisms, or left to sink downward to the abyss. This organically rich muck supports untold numbers of species whose diet thrives on the crumbs and the leftovers that have settled into the mud of the seabed.
Here's an example of a bizarre creature that has been seen thriving on the sea floor foraging on fallen whale carcasses - a giant isopod (Bathynomus giganteus, in scientific parlance). These are crustaceans (like crabs and shrimp) that are very closely related to woodlice and pill bugs you see on land. The terrestrial versions can only grow up to 5cm long. These deep sea giants have been found up to 36cm (14in.) long. They are one of several species of deep sea organisms that are inexplicably large. Scientists call this propensity for largeness gigantism and are currently unable to explain why some creatures of the deep grow so much larger than their shallow-water relatives. See what other giants live in the deep ->
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Here's an example of a bizarre creature that has been seen thriving on the sea floor foraging on fallen whale carcasses - a giant isopod (Bathynomus giganteus, in scientific parlance). These are crustaceans (like crabs and shrimp) that are very closely related to woodlice and pill bugs you see on land. 
The terrestrial versions can only grow up to 5cm long. These deep sea giants have been found up to 36cm (14in.) long. They are one of several species of deep sea organisms that are inexplicably large. Scientists call this propensity for largeness gigantism and are currently unable to explain why some creatures of the deep grow so much larger than their shallow-water relatives.