Our planet is called 'Earth,' but a more accurate name might actually be 'Ocean'! Oceans and seas cover over 70% of the planet's surface, but because they support life at much greater depths than land, they actually make up 99% of all the earth's habitable space. The oceans are also what makes land habitable for us, even if we don't live on the coast. For example, the rain that falls on your town evaporated from the ocean and half of all the oxygen you breathe is produced in the oceans. (They are full of microscopic plants called phytoplankton that produce oxygen by the process of photosynthesis.)
If you've ever spent time at the beach, you know that the ocean is immense and powerful. Have you ever wondered what it's like deep below the surface? What the land looks like at the bottom? What kind of creatures live there? And what it would be like to go see for yourself?
If you're swimming at the beach, you aren't standing on the ocean floor. The land that juts out from the edges of each continent is called the continental shelf. This shelf can be very short, or it can extend hundreds of miles out into the ocean. The continental shelf drops off down a steep continental slope which leads down to the real ocean floor, called the abyssal plain. The abyssal plain has its own stunning geography: the tallest mountains, longest mountain ranges, and deepest canyons in the world.
Along underwater mountain ranges are openings in the ocean floor that spew out superheated water with toxic concentrations of minerals. These underwater geysers are called hydrothermal vents. Ocean water seeps into cracks in the ocean floor and becomes heated by the magma (liquid rock) beneath the earth's crust. The hot water (sometimes 750°F) pours back out of the vents, and as it meets the cold 35°F deep sea water, its dissolved minerals precipitate out - become solid again. These minerals often build up to form towering chimneys. The 'Godzilla' chimney off the coast of Oregon grew to be as tall as a 15-story building before it fell over! It is now rebuilding again.
Deep Sea Creatures
You might not expect anything to live near hydrothermal vents. In the first place, they are usually more than 7,000 ft (1.3 miles) below the surface. At this depth the water pressure is extreme - it would crush us in no time at all! In addition, the temperature near a vent fluctuates drastically between hot vent water and cold sea water. There is no natural light this deep in the sea; it is darker than the darkest midnight on land. The water spouting from the vents is full of toxic chemicals and minerals. How can anything live in such a harsh environment?
Despite the odds, hydrothermal vents support a lot of life! They teem with 5-inch-long white vent crabs that eat deep sea worms and mussels. These crabs are designed to withstand the tremendous water pressure. When scientists bring them up to the surface to study them, they die unless they are put in pressurized tanks. The ocean floor has too much pressure for us, but the surface doesn't have enough pressure for vent crabs!
Hydrothermal vents are also home to fields of 8-foot-long tubeworms, worms with bright red plumes that live in tough white tubes made of chitin, the same substance crab shells are made of. Tubeworms have no mouth or stomach, so how do they stay alive? They host bacteria inside their bodies that convert the toxic chemicals from the vents into food. This conversion process is called chemosynthesis, and is unique to the vent ecosystem. Life on land and in the upper reaches of the ocean is supported by energy from the sun through photosynthesis. No sun can reach the bottom of the sea, so chemosynthesis must take its place for many deep sea creatures to survive.
Other creatures live deep in the sea beneath the reach of sunlight. Many of these survive by creating their own light through a chemical reaction. This type of light is called bioluminescence. Bioluminescent animals use their light to find food and mates. Some species, like the anglerfish, dangle a lighted 'lure' from their bodies. Other fish think this lure is food, but when they try to eat it, the anglerfish eats them instead! (Click here to see a picture of an anglerfish.)
Exploring the Deep
There is only one way to explore the very bottom of the sea - in an underwater craft called a submersible. Humans can dive down to depths of 130 feet if they use SCUBA equipment (self-contained underwater breathing apparatus). With extra special equipment and training they can go down to 300 feet. But the average depth of the ocean floor is around 12,000 feet and in some places much deeper!
Exploration with submersibles began with vehicles called bathyspheres in the 1930s. These were simply steel balls lowered from a ship with two people inside. They couldn't maneuver on their own, and they couldn't collect samples. To overcome these limitations, bathyscaphs ('deep boats') were developed. The most famous bathyscaph is the Trieste. While it was large and not very easily maneuverable, the Trieste made the deepest dive in history: in 1960 it dove to 'Challenger Deep,' the deepest part of the Mariana Trench near Japan. The water pressure at this spot is eight tons per square inch! No one will break the Trieste's record of diving 35,810 feet (almost seven miles) because that happens to be the deepest point on the planet. If you set Mt. Everest down on the bottom of Challenger Deep, there would still be more than a mile of water above it!
Five years after the Trieste's deep dive, the 'deep submergence vehicle' Alvin made its first dive. This craft has three windows and can carry two scientists and a pilot. It is easy to maneuver and is fully outfitted with lights, video cameras, clawed arms, temperature probes, and more. Since 1965 Alvin has made more than 4,000 dives carrying scientists that study the geology, biology, and chemistry of the deep ocean. This submersible discovered life at hydrothermal vent sites almost 30 years ago, recovered a hydrogen bomb accidentally dropped in the Mediterranean Sea, and has explored the wreck of the Titanic. It continues today to study the creatures of the deep. Read about an expedition the Alvin took in 2000 and see videos and pictures of hydrothermal vents at this Voyage to the Deep website.
In spite of exploration with manned submersibles like Alvin and unmanned remote-controlled submersibles, the deep ocean is still a vast territory of unknowns. There is work to be done for deep-sea explorers for many years to come!