Man has been exploring the mountains of earth since the beginning of civilization. Yet, many of the mountain ranges of the ocean dwarf any of the ranges found on land. St. Thomas is part of a chain of mountains along the colliding ridge between the North American plate and the Caribbean plate. St. Croix is a lone mountain top about 40 miles south of St. Thomas. The ocean floor between St. Thomas and St. Croix drops to a depth of nearly 5000 meters (2.8 miles!). The drops from the Puerto Rican northern shores to the bottom of the Puerto Rican Trench and from the Continental Shelf (eastern US shores) to the bottom of the Marianas trench are even more dramatic. There is no place on land where you will find such dramatic changes in elevation. With over 70% of the earth covered by sea, there's a lot to be explored! But you must be prepared for the extreme difficulties found below the surface of the sea.
SCUBA
An experience snorkler can dive to depths of 30 meters or more for short periods of time. But if you wish to spend time there exploring, you need oxygen. Running a snorkel from the surface to anywhere significantly below the surface just won't work. At one meter, the net force on your chest resulting from the pressure difference between inside your lungs and outside is around 100 lb! Breathing with even this small of a pressure difference is almost impossible. Humans simply don't have the musculature to do it.
SCUBA (Self Contained Underwater Breathing Apparatus) is system in which a pressure regulator feeds air from a high pressure tank into your lungs at the same pressure as the water around you. With a near zero pressure difference, the diver can breath normally. At least to the diver it feels like normal breathing. In fact, the increased absolute pressure is very important.
The dangers of deep sea diving are well known by experienced divers. The first concern is that of coming to the surface too quickly from a deep dive. Such a rapid ascent has two primary dangers. The first is most likely to occur if a diver panics and swims to the surface without exhaling during the ascent. The higher pressure in the lungs can cause air to be pushed into the surrounding tissue creating painful air bubbles in the tissue or causing a potentially fatal embolism in a blood vessel.
A second danger of rapid ascent, which is far more common, is called the bends, Caisson's disease, or decompression sickness (DCS) . DCS can occur whenever the external pressure drops too quickly, causing the nitrogen normally absorbed in the blood to literally bubble out. To help prevent DCS, some divers use Nitrox , which is a nitrogen/oxygen mixture that is higher in oxygen content than normal air. This reduces the problem by reducing the amount of nitrogen in the bloodstream.
There is another concern that results from prolonged deep dives, rapture of the deep or nitrogen narcosis. At depths of about 100 feet or more, the increased concentration of nitrogen in the bloodstream effects some divers in a manner similar to alchol. Although the malady is not particularly dangerous by itself, it can cause divers to make poor judgements or to act irrationally. Such behavior at 100 feet is always dangerous.
The time that a diver can spend underwater is ultimately limited by the amount of oxygen he/she can carry. But clever new closed circuit rebreather scuba units (like the one shown above) are on the market. In these units, the carbon dioxide is removed from exhaled air, which is then recycled. This reduces the amount air that has to be carried and has the side benefit of fewer bubbles and less noise, a plus for researchers trying to approach skittish marine animals. (For those avid divers among us, a history of diving can be found in SCUBA DIVING EXPLAINED by Lawrence Martin, M.D.)
SUV's
The record for a free dive is held by Francisco Ferreras at 417 feet. The deepest SCUBA dive is only about 60 ft deeper than that! But most work done below 100 ft is left to submarines, bathyspheres, and robots. The deepest dive by any humans was done in 1960 in a bathyscape called Trieiste. Jacques Piccard and Donald Walsh rode in the Trieste to the bottom of the Marianas trench at a depth of nearly 36,000 feet. (If you are interested in just how deep man and other various creatures go, check out Smithsonian's deep dives site.) Because of the inherent dangers with deep water diving and the need for long term data collection and monitoring, a great deal is being done with autonomous underwater vehicles (AUV's). Check out the latest AUV's being built by the MIT Sea Grant program. Their newest AUV, the Delphini, can dive to depths of 3000 m for up to 20 hours. If you are interested, check out the University of Hawaii's list of institutions involved with underwater marine biology research being done with AUV's.
courtesy of MBARI
Be sure to enter your full name. Enter your email address ONLY if you want the number of points awarded e-mailed to you. Responses should be brief but complete. You must click the SUBMIT button to submit your entry. Entries are due by noon on Monday of next week.
Questions:
Bout how long after diving should you wait before flying in an airplane?
What is the primary danger to using Nitrox mixtures?
For how long should you abstain from alcohol before a dive to reduce the probability of nitrogen narcosis.
What is one of the chemicals that removes the carbon dioxide in the rebreathers?
For how long can MIT's workhorse, the Odyssey IIc, operate?
An experience snorkler can dive to depths of 30 meters or more for short periods of time. But if you wish to spend time there exploring, you need oxygen. Running a snorkel from the surface to anywhere significantly below the surface just won't work. At one meter, the net force on your chest resulting from the pressure difference between inside your lungs and outside is around 100 lb! Breathing with even this small of a pressure difference is almost impossible. Humans simply don't have the musculature to do it.