The team is on a mission (I think it was supposed to be a super easy mission, Tony just had very bad luck) and Tony end up falling into a mine shaft somehow. Rocks end up collapsing on Tony and he ends up pretty badly injured. He can't contact the team because there is too much stone between them. I think he ends up arguing with Jarvis over his injuries, i.e. he's hurt and doesn't want to move because ouch, but Jarvis fusses at him until he does, because otherwise he'd have bled out and died.
The team wants to go after him immediately, but they find out the mine was abandoned due to bad instability. It drives them crazy, but they have to wait for experts and locals to show up to help them out. Pretty sure it takes a couple of hours before they're allowed to enter. I remember Natasha plays a pretty big part in exploring the mines to find Tony. I also remember Bruce is in the mines, but I don't remember if he helped explore or if he was there strictly for medical reasons, as they don't know Tony's condition.
When the team (at least Steve, Nat, and Bruce, I think) does finds him, he's either unconscious or going in and out of consciousness. They end up having to drag him out of the mine very carefully. At this point, all but Tony's helmet has been ditched (It's helping him breathe, I think.) I remember a scene where they have to go through a very tiny tunnel and have to very carefully navigate unconscious Tony through it. I'm pretty sure the helmet gets stuck here, though I don't remember if they have to get rid of it at this point, or if it just takes them a long time to get him through it.
Tony lives, though he very nearly doesn't.
Pretty sure the P.O.V. alternates between Tony and Steve, with maybe a little from the other team members. Also that there is strong Tony/Steve feelings, though I don't remember if it was couple-feelings or best friend-feelings.
I'd really appreciate anyone who could help me. I've been driving myself nuts trying to find it.
Found by the lovely Piplover! 450 Feet Under by Veldeia