system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are tunnels that go underwater built?

I don't understand how they are built underwater at all especially the older ones.
gpt
Under the water is some kind of ground.  If you're lucky, that ground is sturdy clay or chalk that's easy to mine through by digging underground, and holds its shape long enough to quickly reinforce the tunnel with brick walls or with iron rings.  If you're not lucky, that ground may be unstable silt, or really hard granite.

In some situations, a river can be diverted through a temporary channel and the tunnel can be dug and covered over with a brick or concrete roof while the riverbed is dry.  Sometimes you can do half the river at a time.

For some crossings, it turned out to be easier not to tunnel at all, but just to put down big pipes at the bottom of the bay or river, hook those up at both ends to the approaches, and pump out the water.