Dreams of travelling through tunnels have deep significance in Freudian psychoanalysis. When they appear in game motifs, though, they mean nothing. Nutting, I tell you.
Apart from the realization that, if you were flying a missile, you wouldn't necessarily want to fly it into a long tube, steering through rotating locks. You'd think that that'd be obvious, but I thought I'd mention it, anyway.
The little thing in a pool of blood is called a "Puki." They are so cute waddling towards you with the aim of ripping your face off. There's some kind of a backstory about them, but I frankly don't believe a word of it.
All I know is that you should kill them before they kill you.
Warning: Some sound effects in both.