Fluid immortality, a poem by Robert Krulwich
When the storm passes, you'd think the water would calm, settle and return to a quiet equilibrium, but the energy, oddly, doesn't dissipate. The storm has become a wave that now lives in a patch of sea, moving along with no need for a push from above. It is, says Pretor-Pinney, what scientists call a "free wave," no longer driven by wind (those are "forced waves"). Now it is a moving bit of history, an old sea storm moving on, free to roam. It has become a "swell."