Eccentric treasure hunter Bill Warren intends to find the body of Osama Bin Laden in the North Arabian Sea using sonar and a remotely operated vehicle. If he finds the remains, he'll photograph them to confirm Bin Laden's death to the world. Can you find a human body in the ocean using sonar?
Almost certainly not. Active sonar devices bounce sound waves off whatever objects may lie in their path, then generate images based on how long it takes the waves to return. Soft objects like a human body are difficult to see in these sonar-generated pictures, because sound waves don't bounce off of them very well. A body won't look very different from sand on the sea floor. The technique is much more useful on hard objects, like anchors, cannons, or gold bars. More importantly, unless Mr. Warren has insider information about precisely where U.S. forces deposited Bin Laden's body, the Arabian Sea is far too vast to be scanned in a reasonable time frame. Just like the human eye, a sonar device has to get very close to see small objects clearly.
Legitimate treasure hunters start an active search only after months, and often years, of exhaustive research. If they're looking for a specific sunken ship, they pore over navigation records and data on weather and currents. They use computer software to recreate the ship's likely path. Sometimes, instead of tracking a particular vessel, they search along historic shipping routes. UNESCO guesses that there may be 3 million shipwrecks in the world, and some of those boats went down with millions of dollars worth of gold and silver. Mapping the sea floor along major trade routes can be an effective search tactic, but only if you're looking for something the size of a Spanish galleon.
Once a treasure hunter selects a specific area and launches a search boat, the next step is to lower a sonar-equipped device called a towfish toward the bottom. On the first pass, the towfish hovers about 50 yards from the sea floor. At that level, it can scan an area 650 yards across, excepting a 50-yard slice directly beneath the emitter. A human body would be basically undetectable at that distance, perhaps appearing as a ripple in the sand. Even a shipwreck can be tough to discern for an untrained eye. If the towfish reveals something abnormal on the floor, the operator passes it over the same spot, this time within just a few meters of the bottom. Finally, the ship releases a robotic video vessel to get a better look.
Bonus Explainer: If Warren defies all odds and finds Bin Laden's body, will it be recognizable in a photograph? This turns out to be a difficult question. Forensic anthropology research on body decomposition rates has focused more on shallow terrestrial graves and car trunks than seawater, so there isn't much to go on. The little research that has been done suggests the current state of Bin Laden's body depends on where it landed. If it's in an area with few scavengers, low temperatures, and a limited supply of dissolved oxygen, the decomposition process may have barely begun, and the body could be easily recognizable. Generally speaking, these conditions are more likely in deeper parts of the water, where higher pressures also prevent bloat—but that rule isn't iron-clad.
It's equally possible that Bin Laden has already been reduced to bones. Research on pig carcasses dropped to a depth of around 300 feet shows that marine scavengers such as lobsters, crabs, shrimp, and possibly sharks can dismember a carcass within a few days. (If the bag containing Bin Laden's body is well-sealed, it can help deter the smaller scavengers.) Once the large scavengers have started the process, a wider array of animals moves in. The pig research suggests that a dead body can be picked clean in less than a month.
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Explainer thanks Gail Anderson of Simon Fraser University and Sean Fisher of Mel Fisher's Treasures.