ROME: Modern scientists have long been baffled by the remarkable durability and water-resistant nature of Roman concrete, even after it has been submerged for 2,000 years. Although the exact recipe is lost, the base ingredients of Roman concrete are volcanic ash, lime, and volcanic rock. A new study suggests that when seawater encounters the volcanic material, it causes a chemical reaction that produces a rare crystalline mineral that subsequently reinforces the structure. Therefore, while modern concrete deteriorates quickly in water, Roman concrete actually strengthens over time. —Jason Urbanus