system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are underwater tunnels built?
gpt
There are several techniques, but the most common way is to make concrete rectangles, like a box with no top and no bottom.  Then you lay the rectangles on the bottom of a trench across the river bottom, making a rectangular tube.  Then you bury the tube and pump sealant into the seams.  After the sealant cures, you pump out the water.