It was a British geologist who in the early 1920s discovered that Stonehenge, the mysterious circle of rough-hewn monuments in southern England, was roughly 4,000 years oldfar older than previously thought. He also showed that the stones themselves had been hauled from a great distance200 miles (321.8 kilometers) from a quarry in western Wales.
Later in the 20th century revolutionary dating methods have revealed the ages of artifacts with increasing precision. Satellites and related technology, such as ground-penetrating radar, have helped locate likely sites of human habitation.
Technology has also opened up an entirely new realm: the sea. In the early 1940s Frenchmen Jacques-Yves Cousteau and Émile Gagnan co-invented the Aqua-Lung (known today as scubaself-contained underwater breathing apparatus). Anthropologist George Bass and historian Peter Throckmorton in the early 1960s began using this gear to explore ancient shipwrecks in the Mediterranean.
The late 20th century saw the development of a new generation of undersea explorers: remotely controlled submersibles. They can dive far deeper and stay down far longer than humans. Explorer Robert Ballard has used these devices to solve mysteries surrounding famous shipwrecks, such as the R.M.S. Titanic, and to plumb the depths of the Black Sea for more or less intact remains of ancient wooden sailing vessels.
With the vastness of the ocean floor yet to be investigated for the remains of thousands of years of human seafaring, many predict that the most startling archaeological discoveries of the 21st century will be made underwater.