I just got the game and have been through the training videos.

I'm embarrassed to admit I'm on the first mission, on the beach, and I can't figure out how to squeeze through the narrow opening in the rocks. Is there a key that lets me crawl on hands and knees?

Or what?

Well, I'm not surprised to find that was a really stupid question I asked. But it was only thanks to a visual walkthrough I found that showed me where I should climb the cliff face from the beach. It's certainly not obvious.

But now I've completed the first mission.