How did Led Zeppelin get that massive drum sound on "When the Levee Breaks"?

It was mostly a happy accident. They recorded John Bonham’s drums at the bottom of a huge, echoey stairwell at Headley Grange, with two microphones hung twenty feet up. Engineer Andy Johns also ran the track through a Binson Echorec tape delay and slammed the compression hard. The result is room acoustics, tape delay, and aggressive compression stacked — no studio wizardry, just a lucky stairwell and Bonham hitting hard.