A mole is a massive structure, usually of stone, used as a pier, breakwater, or a causeway between places separated by water. The word comes from Middle French mole, ultimately from Latin mōlēs, meaning a large mass, especially of rock, and it has the same root as molecule. A mole may have a wooden structure built on top of it that resembles a wooden pier. The defining feature of a mole, however, is that water cannot freely flow underneath it, unlike a true pier. The oldest known mole is at Wadi al-Jarf, an ancient Egyptian harbor complex on the Red Sea.