It's 'Whole Lotta Love'.

It’s “Whole Lotta Love.”

Yes, that’s the one — the psychedelic middle section with the theremin swooping up and down. Jimmy Page played it live, too, using a Theremin to make those spacey noises.

The song came out in 1969 on Led Zeppelin II. Page was experimenting with effects, and the theremin solo was his way of adding an eerie, otherworldly texture. It’s not a long solo, but it’s iconic.

If you’re looking for the part: it starts around 2:30, right after the heavy riff drops out and things get weird.