I’m obsessed with a particular musical cue from the Annihilation trailer, and as it turns out, I’m not alone. Since announcing my fascination with the weird little melody, I’ve had several people tell me that they, too, have been haunted by the sound, and the YouTube comments beneath the trailer are full of comments singling it out.

I wanted to find out what makes this noise so compelling, so I called Ben Salisbury, one of Annihilation’s two composers, who explained that the melody I like so much is part of a much longer segment of the score known as “The Alien.” (The sound in question can be heard at the 2:40 mark.)

During our conversation, Salisbury also recounted how he and Geoff Barrow approached Annihilation’s score, how those few notes came to be part of the movie’s trailer, and why they’re so much more powerful when you hear them in the theater.

This interview has been edited and condensed.

Am I the first person to tell you how obsessed I am with this particular musical cue?

You’re not the only one, actually. It seems to have weirdly grabbed people. In fact, I remember way back when we were making the film, people in the editing suite would say they heard people doing this weird little hum, and it was always that same refrain, under their breath.

It’s the only part of our music that’s actually on the trailer. The rest of it is sort of trailer stuff, and then for some reason, they’ve literally just picked that little phrase out, one of the stems from our score, and it seems to have affected people. I don’t know if you’ve seen, but there’s a YouTube channel where two guys and a girl respond to trailers. It’s hilarious. They watch the thing for the first time and they really pick up on that little melody. It freaks them out. I don’t know whether they like it or hate it, but it’s very funny.

People do seem to be divided over whether they like it or find it kind of disturbing, but they definitely notice it. How did this theme come to exist in the first place?

It’s part of a bigger cue that Geoff did, so I’m quite free to talk about it in glowing terms without having to be embarrassed. It was originally a very simple four-note theme that represented The Shimmer [the mysterious area being investigated in the film]. We worked on the film for a very long time and there were probably about five different versions of our score, but at one stage, those four notes of that theme existed throughout the score in an orchestral version. At that point, we weren’t using any synths.

In the final incarnation, it’s still used harmonically throughout in orchestral themes that I did, and it has a sort of musical cousin in a guitar theme, so you’ll hear echoes of it throughout the film. But for the most part, the sound you hear in the trailer now only appears in a very specific part of the film toward the end. There’s an incredibly trippy standout part of the film in the third act that needs to stand apart from the rest of the film, and we thought, let’s bring out the synths and put it through various processes and stuff. That was the result.

Does it have a name? I’ve just been calling it “that weird little sound from the Annihilation trailer.”

It started life as “the Shimmer theme,” but then we decided that The Shimmer would be led by sound design instead, and then the sound got placed on a different part of the movie—if I said too much it would be a spoiler, but it’s where it all comes together and Natalie Portman’s character, Lena, finally comes face to face with what we’ve been searching for.

It went through lots of names, but in that particular incarnation in the trailer, it’s called “The Alien,” actually.

That’s intriguing.

[Director Alex Garland] said, look, this segment is music from start to finish, there’s not a single word spoken, and it goes from strange orchestral pulses into this sort of very involved and heavy electronic workout into a choral thing. And the central piece of it is this strange little sound that you’ve picked up on. Hopefully when you see the movie it’ll make sense. It’s one of the few examples where the use of that music in the trailer could actually benefit your appreciation of the sound when it lands in the film.

What exactly am I hearing in this melody, in terms of instruments?

You are hearing synths, and as I said, it’s the only time in the score that synths and electronics make an overt appearance. They do so purposefully in this part of the film so that it stands apart. It would’ve been done on Oberheim Two Voice and put through various secret ingredients, but it is essentially as simple as it can be. It’s four notes. Sometimes the most arresting things are the simplest things. They’re sort of mangled in the version you hear, but it’s just four notes. It’s the production of it that does it, which obviously Geoff is a master of.

Did you know this sound would wind up in the trailer?

No, I had no idea. Trailers are a funny thing. If I’m being honest, and I think Alex Garland would agree with me and Geoff certainly would, trailers are a mixed blessing for people who work on a film. The studios are in charge of producing them, and they can do what they want. In an ideal world, I think we’d want people to come to the film and not know anything about it. But that’s unrealistic, because you’ve got to sell a film.

As far as trailer music goes, it’s quite rare for the score, or even elements of the score, to be used in a trailer. In Ex Machina, which Geoff and I did, there was a version of the trailer that did use quite a lot of our music, but it’s often farmed out to companies who specialize in doing trailers, and that’s probably why they all sort of sound the same. It’s no fault of the people working on these things, who produce some great sounds, it’s just the expectations of the studios. You know in the trailer when there’s going to be a big rush or a big boom and then a cut to silence. It’d be interesting if trailers used score more often—although often score is still being written when trailers come out. But that was certainly lucky for us that they grabbed that bit.

These few notes are probably the only part of the score that audiences have heard at this point, and they’ve already earned a big reaction. What’s that like for you as a composer?

It’s really gratifying. It comes from the most impactful bit of the score, the part where you’re supposed to notice what’s going on musically and be affected by it. And it’s nice to hear even from the people who don’t like it, who are disturbed by it, because that’s its purpose. It’s nice that it works in the trailer, but I’d love for people to see the whole film and hear if they think it works in the film itself.

It’s a slightly nerve-racking time before a film is released, and we spent a long time on it. It was a massive creative challenge. It’s a real shame that it doesn’t seem to be coming out in cinemas in the U.K.—

Oh no.

Yeah, Netflix have bought it and are distributing it. It’ll likely go straight to streaming, which is a shame, because the impact visually and aurally of the film, especially in the bit we’ve been talking about, is designed to be immersive, which I think you can really only get the full impact of on the cinema screen.

It’s funny that you mention that, because when I talk to people who have seen Annihilation’s trailer in the theater, they know exactly which musical cue I’m raving about. But people who have just seen the trailer online don’t seem to understand what the big deal is.

We did spend a long time mixing it in a studio in London, that particular part of the film, especially. We really went to town on the surround, because the music is really the only thing guiding you in that part of the film.

Anything else to add?

Only that I’ve never been interviewed about four notes before.

Well, I’ve never interviewed anyone about four notes before either. Wait—this cue sounds like it’s five notes to me, but you’re saying it’s actually four?

It’s four plus three, actually. Another one of its names was “the four-note theme,” but I think you can call it five notes, if you want. As I say, it’s been so processed. If you listen to that video of the trailer reaction, they all sing it back to each other, and it ends up just sounding like some sort of demon opera. Maybe that’s what it should be called. The demon opera theme.

Read more in Slate about the weird Annihilation noise.