The Tunnel of Eupalinos is a very special tunnel that was built over 2,000 years ago in ancient Greece. It was built under a mountain to bring water from a spring to a city on the other side.
Imagine you have a big hill, but you need to get water from the spring on one side of it to the city on the other side. What do you do? Well, the ancient Greeks had an idea - they decided to dig a tunnel through the mountain!
But digging a tunnel is not easy. People had to start digging from both sides of the mountain and meet each other in the middle. And this is where the Tunnel of Eupalinos comes in.
There were two teams of men, one team starting from one side of the mountain and the other team starting from the other side. They dug their way towards each other through the mountain, but how did they know where to dig? Imagine drawing a line on a piece of paper and then folding the paper in half so the line meets up perfectly. Then you know that the line is perfectly straight. This is what the ancient Greeks did - they used geometry (another big word that means measuring shapes) to make sure the two tunnels met in the middle.
So, after a lot of hard work, the two teams met in the middle of the mountain and connected their tunnels to make one big tunnel. This tunnel was used to bring water from the spring to the city on the other side of the mountain.
The Tunnel of Eupalinos was a very big and important engineering project in ancient Greece. Today, we can still see the tunnel and even go inside it to see how it was built. It just goes to show how smart and creative ancient people were!