FOUND!!! Sam/Dean, Mermen in Atlantis fic

[dee001]

So, Hi! I'm looking for a fic I read some years ago: Dean(?) discovers a hidden trapdoor in the Bunker basement and when he opens it Merman Sam comes through.... Quite a long fic that ends with Dean going under water to live with Sam and getting changed too. They have a kid (hatched from an egg, I believe) and there's a scene where they visit the hatchery to check on their egg's progress (I think?)
Anyway, if this jogs anyone's memory please, PLEASE let me know! It's positively haunting me!!
Hugs, Dee