Most of the Earth's surface is underwater, where our lungs and fragile bodies can't take us. Here are the manned and robotic vehicles designed to withstand the extremes of cold and pressure so we can explore it.
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The exploration of the underwater wreckage of the RMS Titanic by Dr. Robert Ballard and the subsequent James Cameron film Titanic introduced the world to the technology and the challenges of underwater exploration, but the Titanic lies just 3,800 meters below the ocean's surface. Almost 47% of the ocean lies beyond 4,500 meters.
Sunlight and the heat it creates is almost nonexistent at 200 meters below the surface. A Seawolf class nuclear sub can't extend past 600 meters. Once you hit 4,000 meters the temperature is nearly freezing. At the lowest depths below 6,000 meters, in the hadalpelagic zone, the pressure is eight tons per square inch. To operate at these depths you need vessels capable of working with almost no heat, no light, and crushing pressures. There are numerous classifications, but these manned and unmanned vehicles can be generally divided into three categories.
The bathyscaphe is like a submarine but it lacks any propulsion system and utilizes buoyancy and gravity for movement. The most famous bathyscaphe is the Trieste, which took its inventor Jacques Piccard and Lieutenant Don Walsh near the bottom of the Mariana Trench (10,911 meters) on January 23, 1960. It's the only time humans have ever visited that depth and it was accomplished using two giant tanks of iron pellets fbecause a normal tank couldn't release the water ballast.
The Deep-Sea Vehicle combines the maneuverability of a submarine with the strength of a bathyscaphe and many can reach between 4,500 and 6,500 meters below the ocean. The most famous of these vehicles is Alvin, which Dr. Ballard used to explore the Titanic. It has a crew of three (a pilot and two scientists), two robotic arms for working with tools, video cameras, and bright lamps. Most importantly, though a DSV still uses gravity/buoyancy to move vertically, it has six reversible thrusters to move around. Newer vehicles like the Mir and Nautile are capable of going as deep as 6,000 meters.
Remotely Operated Underwater Vehicles
The most common underwater vehicle is the unmanned ROV. These vehicles vary widely in depth rating, size, and purpose. The majority of the work being done below the surface on the BP oil spill live feed is being done by ROVs. Some are as small as a lawnmower while others are as large as a Honda.
The largest and most advances ROVs are the "heavy workclass" models like the Hercules used by the National Oceanic And Oceanographic administration and the Institute For Exploration, which are tethered to mother ships and can explore the ocean floor at around 4,000 meters. They're made of a special syntatic foam and can carry robotic arms as well as specialized equipment. A specialized ROV called the Kaiko was launched by Japan and combined the properties of a Bathyscaphe and an ROV to make the deepest unmanned dive at 10,911.4 meters.
For more information on underwater exploration check out Gizmodo's celebration of Jacques Cousteau's 100th Birthday.