OK, first of all, just walking through a submarine, say, the S.S. Pampanito (moored just outside the harmless Musee Mecanique) is scarier than a horror movie.
After you crouch through around five of those metal doorway hatches, realizing each time that you can't go back the way you came, that's when you start to feel the whole submarine roll from side to side. You can't see anything move, because you're in a submarine, and everything's nailed down. Your stomach reports back that your eyes are stupid: It's moving! Get out of here! Barf or faint, don't barf or faint, it's up to you. Reach the other end of the beast, and face a cheerful docent showing you the very cool-looking mechanically driven killing machines: the torpedoes. Each weighs one ton and can swim five miles just to blow your butt off. Sure, they're made of insane copper pipes and levers and air compartments, all shined up military-pretty, but it hits your they're pure evil. This doesn't help with the claustrophobia motion-sickness panic attack you're already having. Highly recommended! Must see to believe! Awesome!
Speaking of which, the S.S. Jeremiah O'Brien is siiiiick. All Iron Man and Hellboy fans wish SO MUCH they got to see the Steam Engine Room in motion this weekend. Because: Climb down some sketchy stairs, the air getting hotter with each step. Resolve to heed the many, many frighteningly worded warning signs. Are the handrails too hot to touch? Doesn't matter, you have to hold them, or you'll fall into the … mommy, help! What is that? It's the size of an elephant, the color of overheated car guts, it's moving, and it's underneath you. Keep going!
The place was rigged for civilian visitors in a different time, when the philosophy must have been "Gonna lose a couple clumsy ones anyway." Reach your below-the-water-line destination and look up at the menacing, two-story pistons like the caveman in 2001: A Space Odyssey. Don't touch anything, because you don't know what's full of pressurized steam, now do you? Fires show through tiny windows, and sarcastic plaques point out the 2500 HP steam-powered washing machine; "Do lingerie at your own risk." Hey! Don't joke! I'm scared and there are sugar-drunk children all up in here, lurching every which way with their tender little fingers! More jolly retired military men explain everything except why humans need to make war and why one of the sixtyish coverall-wearing technicians has tattoos all over his face.
Here is a video of the engine room which makes everything look fifty times smaller than it does in real life.
All of the above was part of the Mechanicrawl of the Long Now Foundation.