system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] How were those giant underwater hangers built?

I mean clearly they weren't built from under the river and up.  I also imagine that damming that large of a section of a river would not go unnoticed. How secret were they?
gpt
Why couldn't they be built underwater without damming it?

First step is to excavate an antechamber, something small to stage the hangar doors in.  

Next, build the doors under the surface.  Once they're in-place, remove the material above.  If they leak, only the antechamber floods.  Seal over the door, empty the room, and fix it, then try again.

Once the doors are good, dig out the rest.  Start at the top, adding temporary reinforcement as you go, then add permanent reinforcement once it's done.

This is a big undertaking, but not beyond SHEILD's capabilities, especially if someone like Tony Stark is lending a hand with specialized suits.  Which he has in spades.