Grand Canal — Venice, Italy

More than 170 buildings line the banks of Italy's most famous canal, many of them built between the 13th and 18th centuries.

"Venice has sunk 9 inches in the last century and currently floods about 100 times a year — a phenomenon called the acqua alta," de Lancie said. "Now, after much research, underwater 'mobile' gates are being built on the floor of the sea where the open sea enters Venice's lagoon. When the seawater rises above a certain level, air will be pumped into the gates, causing them to rise and shut out the Adriatic."