1 oblong seedless watermelon (about 14 pounds)
3 cups seltzer
Cut top third off watermelon; trim bottom just enough to make a flat surface. Scoop out flesh.
Working in batches, puree watermelon in a food processor. Strain through a fine sieve; discard solids. (You should have about 9 cups juice.) Cover rind with plastic wrap. Refrigerate juice and rind until chilled, about 1 hour. Combine juice with seltzer; transfer to rind.
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