From Science Now:

For a few months in history, world domination depended on a bridge built upon sand. To conquer the almost impenetrable island of Tyre in 332 B.C.E., Alexander the Great had his engineers construct a land bridge across a kilometer of open water. Given that his army had only basic tools and was under constant attack, the feat seems almost miraculous. But was it? After analyzing the past 10,000 years of coastal sediment deposits, a team of researchers concludes that Mother Nature--not the renowned Greek military commander--was the primary bridge builder.

Alexander was the first commander to attempt to conquer the known world, and his army had just captured the Phoenician cities of Byblos and Sidon. In nearby Tyre, he saw a strategic outpost that would give him a supply and reinforcement port to control the Eastern Mediterranean. But Tyre proved a tough nut to crack. Besides being protected by 50-meter-high walls, the ancient city occupied an island a kilometer off the coast of present-day Lebanon, surrounded by seas as deep as 10 meters. History records that, after seven months of battle, Alexander's army breached the island's defenses by constructing a bridge of timber, stone, and rubble and then used battering rams to puncture an entryway into the cities' walls--a feat that effectively led to the end of the Phoenician Empire.

But just how impressive was this achievement? Geoscientist Nick Marriner and colleagues at the European Center for Research and Teaching on the Geosciences of the Environment (CEREGE) in Aix-en-Provence, France, studied sediment records off the coast of Lebanon and microfossil evidence from core sites on the Tyrian peninsula. The team concludes that at no point did Alexander's engineers contend with anything close to 10 meters of water. Instead, an outpouring of sediment over 5500 years from the nearby Litani delta formed an underwater platform between the mainland and Tyre. As the rise in sea level slowed and agriculture developed, sedimentation rates increased. In addition, Tyre acted as an immense shield to quash waves, allowing material to accumulate on its Lebanon-facing side. By 332 B.C.E., the bridge was within one to two meters of mean sea level, the team reports online this week in Proceedings of the National Academy of Sciences.

Jean-Daniel Stanley, geoarchaeologist at the Smithsonian in Washington, D.C., says the formation of the natural underwater bridge isn't surprising, given what is known about sediment flow. But that's not to dismiss Alexander's accomplishment, which set the stage for his continued conquests before his death at age 32. Considered by some historians as his greatest military achievement, the story of Tyre--and the legend of Alexander the Great--might have read quite differently without an assist from Mother Nature.