The Leslie speaker is the secret.

The swirling vortex sound in “Echoes” comes from running a piano through a Leslie rotating speaker cabinet—the same kind used for Hammond organs. They miked it in stereo and panned the two channels to create that Doppler-effect swirl.

Richard Wright played a simple chord sequence on an electric piano, and the Leslie did the rest. The band also fed the piano through a wah pedal and reversed some tape, but the Leslie is the main ingredient.

I think the effect is so iconic because it’s basically an organ effect applied to a piano. That mismatch makes it sound otherworldly.

If you want to try it yourself, any rotating speaker simulator plugin will get you close. But nothing beats the real thing with a tube amp.