Would-be warlords, take note. Researchers say they have figured out how the Macedonian conqueror Alexander the Great was able to build a nearly kilometer-long road over the sea to strike at the island of Tyre in 332 B.C. Based on geologic samples taken from the area, in what is now Lebanon, they conclude that the island and shore were linked by a stretch of sand a few meters below the water's surface—well-suited for traversing with an artificial bridge.

The island today is in fact more of a peninsula, connected to the coast by an outgrowth of sand called a tombolo, says geoarchaeologist Nick Marriner of the European Center for Research and Teaching on the Geosciences of the Environment in Aix-en-Provence, France. But researchers had never studied the tombolo's history to see if it might shed light on Alexander's attack on Tyre, he says.

On his way through Persia and Egypt, Alexander marched along the Phoenician coast, swiftly taking control of all the major cities except the island city-state of Tyre—then a major trading outpost—which refused to surrender. "All previous settlements on his journey from Macedonia had capitulated with little resistance," Marriner says. "Of course, these all lay on land."

Alexander's armies laid siege to the island for seven months. At some point during the siege—the timing is not clear, Marriner says—his engineers built a long causeway, or raised road, from which to strike at Tyre's defenses. How they were able to build the presumably timber and stone road was a mystery, he says.

Looking to geology for the answer, Marriner and colleagues analyzed long cores of sediment they drilled from the modern tombolo. They conclude that more than 8,000 years ago Tyre was an approximately six-kilometer-wide island, but that between 6,000 and 8,000 years ago sea levels rose enough to shrink its size to four kilometers.

The newly sunken, inland part of the island made it harder for waves from the sea to get past the island to the shore, Marriner and colleagues report in this week's Proceedings of the National Academy of Sciences USA. As a result, sediment from the coast was able to accumulate in the space between it and the island. "It meant," Marriner says, "that the causeway foundations could be laid down in relatively shallow water."