zwischen is between.
It's not a coincidence that zw shows up in both zwischen and zwei, or "tw" in both "between" and "two" -- it's used when something is between two things.
"between" and "among" are thus also rather different things.
For "among", use unter.
Context will let users understand whether unter means "under" or "among", much as native English speakers can tell the difference between the meanings of "on" in "He gave a lecture on a table" and "He gave a lecture on physics".
I agree I can't translate literally, but this phrase was written in an episode where Mulder travels to the past: http://en.wikipedia.org/wiki/Triangle_%28The_X-Files%29. Anyway, I just posted because "Sie sind unter uns" needed this joke :-)
I assume that "uns" is the dative here, not the accusative. Am I right?
"Zwischen" is much more literal and physical than "unter". They're generally not interchangeable. You use "unter" for among / between when it's referring to group membership rather than physical position.
"Sie sind zwischen uns" would mean rather that they are physically located between you and me, and "Let's keep this between us." is translated using "unter", like "Das bleibt unter uns."
Wait, unter means under and among at the same time?
No, not at the same time, but at different times.
In any given sentence, it will mean one or the other depending on what the speaker wants to say.
That does not make any sense!
Prepositions in general don't "make sense".
Why do you "listen to music" but "look at a painting"? What not "listen at music" or "look to a painting"?
Why do we meet "at Christmas in December on a Wednesday"? Why "at midnight" but "in the evening"? Wouldn't it be simpler if the same preposition worked for all of them?
Why not "They are below us!" ?
That's another possible answer, and is accepted as well, but is a less likely interpretation of unter uns than "among us".
We don't often talk about someone being below us but we talk about someone being among other people more often.
Why not? "They are among us" only reminds me of a horror movie. I use the sentence more often in the meaning of "They are below us". Sie sind (eine Klasse) unter uns. Sie sind (ein Stockwerk) unter uns. Sie sind (in der Highscore) unter uns. Sie sind (an der Kletterwand) unter uns. And there are many more.