Tunnel of Eupalinos.

https://en.m.wikipedia.org/wiki/Tunnel_of_Eupalinos

Reading about Pythagoras, who was from Samos, I learn about the tunnel through a mountain they made 2500 years ago. Because the tunnel is small, the excavation rate was limited to the pace of two people. You’d be able to go twice as fast if you came from both directions and met in the middle. But does that seem realistic to you, based on what you know about what they had then?

Indeed it works!

The particular details of the methods are interesting. If you aren’t already familiar, you may enjoy thinking about the problem and how you’d do it.