One of the most impressive engineering accomplishments of the ancient world, this 1,000m (3,280-ft.) tunnel through the mountain above Pithagorio was excavated to transport water from mountain streams to ancient Vathi. The great architect Efpalinos directed two teams of workers digging from each side, and after nearly 15 years they met within a few meters of each other. You descend into the tunnel, and then if you can muster the courage to squeeze through the first 20m (66 ft.) -- the tunnel is a mere sliver in the rock for this distance -- you'll see that it soon widens considerably, and you can walk another 100m (328 ft.) into the mountain. Even though a generator supposedly starts up in the event of a power outage, you might be more comfortable carrying a flashlight.