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Soulsand is in the green circle, the block where to place the water source is in the red circle.
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After you placed water, bubbles make their effect
[media]When you destroy the source block, it creates another water source down.
Sorry for my english.
Confirmed for MC 1.13-pre5.First of all, I'm not sure if this is a bug or a feature! I made an hyphotesis about this bug: The bubbles push you up, then maybe it pushes the water up too creating a new source of it.