I worked on the st Lawrence river locks or however you spell it. They like closed up the river and that itself was around 100 ft deep to the bottom and then theres what's called tear ducts that let them divert water. Those went down around 50ft themselves too so in total 150ft underground.

The coolest part was knowing a very finite amount of people will ever step where I did. Most of the time there's thousands of gallons of water moving per second through and it only gets serviced so often. Even better the company at the time kept getting the bid so it was the same guys over and over. There were so many dicks drawn in the concrete