In the radio version of this episode (but not in the TV version) Zaphod asks for an atmosphere check of the planet and is told "It's okay, but it smells a bit".
The Infinite Improbability Drive from The Hitchhiker's Guide to the Galaxy now has its own entry in Wikipedia.
In a nutshell, the drive enables much faster-than-light space travel and is based on quantum theory. A sub-atomic particle is likely to be in a particular place, but there is also a possibility of it being found light years away. Thus, matter can travel from place to place without passing through the intervening hyperspace, by means of controlling probability.
After Arthur activates the Infinite Improbability Drive, the travellers are seen lying in a heap on the floor. But this tangle of four heroes, if examined closely, proves to have five pairs of legs sticking out of it, rather than four. The extra legs are those of a stage hand who was attached to Zaphod Beeblebrox, providing his third arm.
Zaphod: What does the Z mean?
Trillian: Which one?
Zaphod: Any one.
Arthur Dent: The Earth!
Slartibartfast: Well, the Earth Mark II, in fact. We're making a copy from our original blueprints.
Arthur Dent: Are you telling me you originally *made* the Earth?
Slartibartfast: Oh, yes. Did you ever go to a place - I think it was called Norway?
Arthur Dent: No. No, I didn't.
Slartibartfast: Pity. That was one of mine. Won an award, you know. Lovely crinkly edges.
Zaphod Beeblebrox: Okay. So, ten out of ten for style, but minus several million for good thinking, huh?
Marvin: Life. Don't talk to me about life.
Eddie: Hi there. This is Eddie, your shipboard computer, and I'm feeling just great, guys, and I know I'm just going to get a bundle of kicks out of any program you care to run through me.
Zaphod Beeblebrox: Computer, if you don't open that exit hatch pretty damn pronto, I shall go straight to your major data banks with a very large axe and give you a reprogramming you will never forget, capisco? (pause) Okay. Get the axe.
Slartibartfast: It is most gratifying that your enthusiasm for our planet continues unabated. And so we would like to assure you that the guided missiles currently converging with your ship are part of a special service we extend to all of our most enthusiastic clients, but the fully-armed nuclear warheads are of course merely a courtesy detail. We look forward to your custom in future lives.
Arthur Dent: I wonder what will happen if I press this button?
Ford Prefect: Don't!
Arthur Dent: (presses it) Oh.
Ford Prefect: What happened?
Arthur Dent: A sign lit up saying "Please do not press this button again."
Book: It is an important and popular fact that things are not always what they seem. For instance on the planet Earth, man had always assumed that he was more intelligent than the dolphin because he had achieved so much… the wheel, New York, wars and so on… whilst all the dolphin had ever done was muck about in the water having a good time. But conversely, the dolphins believed they were more intelligent than man, for precisely the same reasons.
Book: So long, and thanks for all the fish.
Human intelligence - historical allusion.
This concept is illustrated by a series of pictures of human behaviour including war. One of which is based on Eddie Adams' famous photograph of the execution of a Viet Cong guerilla in 1968.