While there are very few places left on the surface of the earth that have not been explored, most of the ocean has not been visited by humans – mainly because of its depth and inaccessibility.
The average depth of the ocean is more than 3,657 meters (12,000 feet) and the deepest spot in the world is the Marianas Trench in the Pacific Ocean, which is 10,911 meters (35,798 feet) deep.
Scientists divide natural objects and living things into several categories—animals, plants, fungi, bacteria, and protists.
Corals (and the algae that live in the corals) can only grow in a few regions of the world where the water is shallow enough for sufficient sunlight, where there is not too much silt in the water, and where the water temperature is warm enough---but not too warm.
Coral reefs are only found in tropical and sub-tropical waters around the world.