#undertide__undercurrent  a current below the surface of a fluid
  supertype:  #current__stream  a steady flow (usually from natural causes); "the raft floated downstream on the current"; "he felt a stream of air"
  subtype:  #sea_puss  the seaward undercurrent created after waves have broken on the shore
  part of:  #tide  the periodic rise and fall of the sea level under the gravitational pull of the moon

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