I think the lung rooms are a spiral shape, the bridges just serve as shortcuts to get into the main room. If you keep rotating around it in the right direction eventually you'll find a permanent entrance deeper inside.

Also, I found out the hard way that walking a mech on the bridges also makes it collapse, haha

Also also, using about half of an autocannon mech's ammo is enough to kill the lung, no hellbomb backpack required ^^ (Even heavy armor pen damages it, so you can kill it with enough firepower from plenty of different weapons)

That's good to know that you can blow it up from inside the water though! I'm sure it'll be useful to know, and I'll test it by pressing the button on a friend's backpack and kicking them in...

I'm hating all the cave levels. Even throwing my drops right in the middle of a room with a huge fucking roof, my shit always hits the cave roof and just wastes the stratgem 😬