Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
Back to Home
Graphical menu
  We want to thank everyone who sent their questions to Ask the Expert. Unfortunately, we could respond only to a limited number of submissions, and we are not accepting additional inquiries. The answers to many questions can be found within the pages of this Web companion piece.

Margaret R. Goud Collins, Ph.D., of Woods Hole Oceanographic Institute, has answered some of the most popular and intriguing questions. You can find her responses below. External links are valid as of August 6, 1999.

Question: What does the deep sea sound like? I have heard humpback whale songs, but was also intrigued by the "background" echoes and other, more subtle sounds of currents, volcanic activity, schools of fish, etc. Are there any Web sites that feature the sounds of the deep (not the seashore), much like there are Web sites that show current satellite views of the Earth? (Or are these ocean sounds and their collection classified as militarily sensitive...?)

Answer: As in air, sounds in the ocean are caused by anything that makes the molecules of the fluid begin to vibrate against each other: rainfall, explosions, calls by whales, earthquakes, fish swimming -- the list could go on forever. Also as in air, the sounds propagate away from the source as acoustic waves. However, in some parts of the ocean, because of the properties of the water column, sounds can travel for hundreds or thousands of miles.

The properties of sound in the ocean, or marine acoustics, have been extensively studied because of the interest of the world's navies in finding, or hiding, submarines. While much of this research was always unclassified, even more has been made public since the end of the Cold War (though, of course, there is still a lot that is secret). In recent years, the field of bioacoustics has flourished as the U.S. Navy has worked with whale researchers to apply recently declassified data and instruments to the study and preservation of marine mammals, and the study of underwater volcanism and earthquakes has also advanced thanks to these data.

The Monterey Bay Aquarium Research Institute has a good explanation of the uses of underwater acoustics, and an excellent sampler of sounds detected under the ocean, including those made by marine mammals, fish, and natural disturbances, as well as unidentified sounds. The sampler of sounds is at

A straightforward explanation of underwater acoustics, along with a number of recordings of whale songs and other underwater sounds, can be found at the Web site for the National Oceanic and Atmospheric Administration's project on Whale Acoustics:

This site also describes the project's approach to the study of whales using acoustics, and has an extensive list of relevant links. These links can lead you to the acoustic methods used to study marine earthquakes and volcanic eruptions, among many other things.

Question: An old SMITHSONIAN article reported episodic waves ranging along the southeastern coast of Africa, from Durban down, as hazards to navigation, able sometimes to swallow large ships whole so that they disappeared without a trace. How frequent is this problem?

Answer: Scientists believe that these "rogue waves" or "freak waves" can be caused when strong, high storm waves slam into a powerful current traveling in the opposite direction. The interaction can push together and superimpose the storm swells, creating one tremendously powerful wave that can reach a height of 100 feet or more. These are the waves blamed for the Waratah disaster portrayed in the SAVAGE SEAS episode "Killer Waves," and are discussed in the "Crow's Nest" section of this SAVAGE SEAS Online, in a sidebar article entitled "Freak Waves." It should be noted that the powerful Agulhas current off the Wild Coast of South Africa can be hazardous even without generating freak waves.

Incidentally, researcher Emlyn Brown's most recent foray in his 18-year quest to discover the fate of the Waratah has borne fruit. According to OCEANSPACE, an online marine science and ocean technology newsletter and magazine (, digital side-scan sonar imagery used on a cruise this year has allowed Brown's crew "to accurately identify the ill fated vessel by diagnostic features on the wreck. These features include the overall dimensions, stern shape, rudder, davit, derrick & air intake funnel positions. The vessel, although intact, has suffered severe structural damage, including 4 hull fractures that correlate to the bulkhead configuration. This damage would probably have occurred when the vessel collided with the seafloor.

"Limited video footage was obtained, which includes the 'champagne glass' shaped stern, rudder and stern davit, which confirms the side-scan imagery. The success of this expedition has cleared the way for a more detailed survey, including the use of a submersible, and/or a diver survey which will be used to video the entire vessel and recover certain artifacts. It is hoped that the second leg of the expedition will shed some light on why the vessel sank with all hands in such a mysterious manner."

Question: In the show which showed the effect of tracking the result of shipping lanes and the effects of the 7-year Pacific Ocean cycle you had someone who was able to show a simulation of the ocean waves and predict where items lost at sea would wash up. I like to find out who and how they did that prediction?

How can you predict where shoes off an ocean liner that has lost some cargo will end up? What types of variables are used to predict this?

Answer: The model that was used to predict where the shoes from the ocean liner would wash up was developed by James Ingraham of the National Oceanic and Atmospheric Administration's Alaska Fisheries Science Center in Seattle, WA. The model was applied by Dr. Curtis Ebbesmeyer to forecast (or hindcast, in the case of the sneakers that washed ashore and inspired the study) the path of the lost sneakers. The two scientists have cooperated on a number of studies that opportunistically use drifting objects that have been accidentally introduced into the ocean's surface waters. (Oceanographers also purposely launch drifters to study surface currents. In the past they would frequently include a note asking beachcombers to mail them a report on where they found a marked drifter; these days, they more often track the drifters by using satellites.)

The study and modeling of the ocean's surface currents is a central theme of the field of physical oceanography, and it's a complicated business. The models used differ depending upon the interests of the person doing the study, for example:

-The scale of interest (i.e., how large an area are you describing -- an ocean basin or an estuary?);

-The physical processes you're interested in (i.e., do you want to know the effect of wind on currents? Or the effect of heating by the sun? Or the relation of currents to the occurrence of particular types of fish?);

-The data you have (i.e., if you're trying to predict the path of a spill of rubber duckies, you have to include the effect of wind on their drift because they stick out of the water, whereas the effect of wind can be neglected in studying the paths of the sneakers).

Such models might include parameters describing the temperature and salinity of the water, the wind, the earth's rotation, the initial velocity of the water, the shape of the ocean basin, and differences in sea surface height. In the case of the drifting tennis shoes, the model would include the date and location of the cargo spill, and it would then follow the path of that patch of water for the next few years. If tennis shoes start showing up at locations or times that you don't predict, then you go back and figure out where the weakness is in your model.

Dr. Ebbesmeyer has set up a Web site ( aimed at beachcombers, asking them to report discoveries of drifting objects.

Question: How cold does the water have to be for you to die in less than one minute?

I go long range fishing out of San Diego California down into Mexican waters for tuna and wahoo. Some trips will go as far as the tip of Baja. In the back of my mind there is always the question, "If our boat goes down and the life boats do not get deployed, how long can I survive in the water for?" So I am interested in finding out if there is some sort of a chart, curve or graphic that shows time and water temperature as it relates to body temperature loss. Please include your personal advice on this subject.

Answer: Cold water in and of itself can't kill you in less than a minute, but the shock of sudden immersion in cold water can cause an involuntary gasp that will drown you almost immediately if your head is underwater at the time.

If you don't drown immediately, immersion in water can cause hypothermia, even in relatively warm water if you're in there long enough. Hypothermia is lower-than-normal body temperature. As core temperature drops, the body responds by shivering (to try to warm up) and cutting circulation to the limbs (so they get numb); this is followed by confusion, then unconsciousness. Death usually occurs when the heart cools to 77 degrees Fahrenheit. In water near freezing temperatures (32.5 degrees or less), unconsciousness usually occurs in less than 15 minutes, and expected time of survival is less than 15-45 minutes. If the water is between 32.5 and 40 degrees, unconsciousness occurs in 15-30 minutes, and death usually by 90 minutes. If the water is over 80 degrees, by contrast, you may be able to stay conscious indefinitely, and a survival suit can slow the cooling process substantially.

There are lots of Web sites that have information on hypothermia and boat safety, including charts showing survival times in various water temperatures. A couple include:, specifically, and for hypothermia information.

Question: Can earthquakes on the ocean floor cause tsunamis, whirlpools, and freak waves? How often do they happen?

Answer: Earthquakes don't cause whirlpools and freak waves, but earthquakes and underwater volcanoes can cause tsunamis. According to the U.S. Geological Survey National Earthquake Information Center (, the following table describes the historical frequency of earthquakes, based on observations since 1900.

Great earthquakes (8.0 and higher on the Richter scale): 1 per year
Major earthquakes (7.0 - 7.9): 18 per year
Strong earthquakes (6.0 - 6.9): 120 per year
Moderate earthquakes (5.0 - 5.9): 800 per year
Light earthquakes (4.0 - 4.9): 6,200 per year (estimated)
Minor earthquakes (3.0 - 3.9): 49,200 per year (estimated)
Very minor earthquakes (2.0 - 2.9): About 1,000 per day
Very minor earthquakes (1.0 - 1.9): About 8,000 per day

Tsunamis can occur anywhere in the world, but they are most frequent on the Pacific Rim because of the large number of active margins of tectonic plates there. (That is, places where two tectonic plates are moving toward each other, or rubbing against each other. It's this movement that causes the earthquakes that set off tsunamis.) Large earthquakes that cause ground motion on the bottom of the ocean can create a tsunami directly by shaking the water column -- this will set up a fast-moving wave that can travel quickly across the whole ocean basin. These waves can be detected using observations by offshore buoys or satellites and often give time for warnings to be distributed. However, even smaller earthquakes, with magnitudes of seven or less, may cause underwater landslides that can generate a devastating tsunami nearby, without time for warnings. The destructive tsunami in Papua New Guinea on July 17, 1998, where waves with heights of 10 to 15 meters wrecked seven villages and killed more than 2,200 people, appears to have been generated by such a landslide, according to a study published in the July 27, 1999, issue of EOS by a multinational scientific team.

A good source of information on tsunamis is maintained by the University of Washington at

Question: I am designing a sailing yacht and wanted to know about the dangers of lightning strikes at sea, and how best to channel lightning without having it damage either the boat, the composite mast(s) or the reasonably sensitive marine electronics and computers aboard.

Answer: A sailboat at sea is obviously not a desirable place to be in a thunderstorm. Lightning is likely to strike the tallest thing around, and that is apt to be your mast. Thunderstorms are common in the warm waters of tropical areas where sailing is attractive, however, so it is important to make your boat as lightning-safe as possible. Doing so requires a thorough grounding of all metal on the boat: make the path to the water from the mast, or radio, or antenna as direct as possible. An excellent explanation of lightning risks at sea, and how to minimize them, is maintained by the University of Florida at

The National Lightning Safety Institute also has other suggestions for boat designers at their Web site:

Question: What was the name of the German ocean liner which sank in 1945 and had the highest loss of life of any shipwreck this century?

Answer: The German luxury liner Wilhelm Gustloff, converted for wartime use as a hospital and barracks, was hit by torpedoes from a Soviet submarine in the Baltic Sea on January 30, 1945, and sank with a loss of life estimated at between 5,000 and 7,000. The ship was a 25,484-ton luxury cruise liner commissioned in 1938, built to accommodate 1,465 passengers and a crew of 400. It was crowded with at least 6,050 persons, according to the ship's manifest, when it sailed from the port of Danzig; most of the passengers were fleeing from the advancing Red Army as the war drew to a close. More refugees reportedly boarded the ship from small boats as the Gustloff left the harbor, so the exact number on board is unknown. After the ship was hit by three torpedoes, it sank rapidly, and only 900 to 1,000 of the passengers were rescued from the sea.

A brief description of this and many more WWII marine disasters can be found at In addition, a memorial page for the Wilhelm Gustloff by a student and staff member at the University of Wisconsin-Milwaukee who is a devotee of German WWII history is located at

These sites are maintained by dedicated individuals and are filled with interesting information. However, I have no insight as to their absolute correctness.

Question: Why do sharks attack people?

Answer: According to shark experts, the reason for attacks on humans by most shark species is defense. A shark will perform a threatening behavior when a diver or swimmer approaches too closely or otherwise provokes it, and may bite. Some larger species attack for feeding, most notably the white shark, the tiger shark, and the bull shark. Provocation is heightened by the kicking or thrashing vibrations people make in the water (which, to sharks, resemble the irregular movements of a wounded fish), the presence of speared fish or bait in the water, or the presence of blood from wounds or menstruation. Sharks have highly developed sensitivities to chemical or electrical disturbances in the water, and may be aroused by even minute concentrations of blood or food. It has been estimated that there are about 100 shark attacks worldwide per year. About 25 percent of these are fatal, largely owing to hemorrhage and shock -- although the bull and white sharks are large enough that they can, and sometimes do, take people whole. It should be noted, however, that shark attacks are much less frequent than other aquatic mishaps.

For more information on sharks, a couple of good places to start on the Web were created in conjunction with two PBS NOVA programs: is the companion Web site for the 1996 NOVA program "Shark Attack!," and accompanied the 1998 NOVA program "Island of the Sharks." In the latter, the "Ask the Expert" column features Dr. A.P. Klimley, an associate research behaviorist at the Bodega Marine Laboratory, University of California at Davis, and I have relied on that column for this answer.

Question: After several hours of searching, I've been unable to find a detailed description of air embolism, the formation of nitrogen bubbles within one's bloodstream. Specifically, how do these bubbles form? Thanks for your time.

Answer: You have described two different conditions that can affect scuba divers. Air embolisms are traveling air bubbles in the bloodstream, and nitrogen bubbles in the bloodstream can cause decompression sickness. Both their causes and effects are distinct, but each can lead to serious discomfort or even death.

Air embolisms are the most serious effect of unequal air pressures that can be encountered while diving. Air that enters a diver's lungs from a tank during a dive is at the same pressure as the surrounding water, so the air is denser than at sea level. If the diver holds his or her breath during ascent, then the dense air in the lungs expands as the surrounding pressure drops, and the lung tissue can tear. This vents air into interstitial space in the lungs, from whence it can enter the pulmonary veins and travel as a bubble, or air embolism, as the blood circulates. These air bubbles can block blood vessels to the brain or heart, and can be fatal.

Decompression sickness -- also known as caisson disease or, more commonly, "the bends" -- is a result of breathing air under pressure. The air we breathe is approximately 21% oxygen and 78% nitrogen, and the amount of each that enters the bloodstream from the lungs is proportional to the air pressure. Therefore, when the ambient pressure doubles, which occurs at only 33 feet below the surface, the amount of oxygen and nitrogen that enter the bloodstream is proportionately greater. Since one atmosphere of pressure is added for each 33 feet, that means that the deeper the diver goes and the longer the dive, the more nitrogen and oxygen enter the blood.

Most of the extra oxygen is metabolized and doesn't cause problems for recreational divers. However, nitrogen is inert; that is, it isn't metabolized. It stays dissolved in the bloodstream until the ambient pressure is reduced and then it starts to dissolve back out. If pressure is reduced too quickly, however, the nitrogen isn't released slowly into the lungs; instead, it can form bubbles in the bloodstream that can lodge anywhere in the body, causing headaches, joint pain, paralysis, or even death.

An excellent explanation of these and other diving physiology questions can be found on the Web in an online book entitled SCUBA DIVING EXPLAINED, authored by Dr. Lawrence Martin of the Mt. Sinai Medical Center in New York City; the address is This book was the source of most of the answer to this question.

Question: In the Savage Seas program there is reference to a point off California (I believe more specifically San Francisco) that was described as the deepest point in the oceans. I was wondering if I understood this correctly or if there maybe another location that holds this designation. In anticipation of your response, thank you.

Answer: I'm not sure what statement you're referring to in the program, but the deepest part of the ocean is the Marianas Trench, in the Pacific Ocean southwest of Guam. The water there reaches a depth of 36,198 feet. More information on the only human descent to the bottom of the Marianas Trench is available in the "Deep Sea" section of SAVAGE SEAS Online.

Question: Why does the human body cool so much faster in cold water, than in cold air of the same temperature?

Answer: Cold water conducts heat away from the body 25 times faster than cold air because water has a much higher conductivity than air. Think of how much faster heat is conducted from a frying pan by a metal handle than by a wooden one -- that's because the conductivity of the metal is so much greater than wood. Body fat can slow the cooling of the body's core when a person is immersed in cold water because it acts as insulation against heat loss; the fat has a lower conductivity than skin and bones.

About the Series | Savage Sites | Facts From the Sea | Ask the Expert | Books & Videos | Credits

ANIMATIONS: Wave Machine | Deep Sea Simulator | Trieste | Satellite Viewer

Crow's Nest | Captain's Bridge | Deep Sea | Weather Factory

Savage Seas Homepage
PBS Online | Thirteen Online

PBS SAVAGE SEAS Thirteen Online