I’m going to play around with this idea because HappyEndings is right that the lake was dried up in Lady of the Lake after the Siren died. But the water was still there somehow/somewhere because Cora activated it and Rumple was able to use it to bring magic.
So I think the water exists in multiple world simultaneously. In mythology, water is always pretty important and has special properties. So I think it can be in our world and in FTL at the same time. It’s a connection between the two which is why it is so effective as a restorer of magic and a portal.
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"He was a lot of things to me" "The only conclusion was love"