Courtesy Harold Edgerton

Underwater archaeologist Elisha Undner employed divers as well as side scan sonar to locate and study the ancient harbor installations at Tharros. Invented by MlT professor Harold Edgerton—seen deploying the torpedo-like sonar into Boston’s Charles River—the side-scan sonar projects its probing sound waves from an underwater point close to the unseen reflecting object. The reflected signal is recorded by the black box on board the boat. Ordinary sonar sends out its sound waves from the ship rather than from far beneath the water’s surface, thus increasing the likelihood—as compared to the side-scan sonar—of interference with, and weakness of, the reflected wave.

Edgerton’s sonar located an anomaly about 15 feet underwater and 100 yards from shore. Undner and his team of divers then investigated and found a major wall that probably served as a breakwater to form a protected harbor.