Mentioned in Ethan's children's books before he arrived in Fromville. S4 made it the show's central physical mystery. Ethan dreamed of it after the S1 RV crash. Jim's ghost in S4E3 told him to find it. By S4E5 it had killed Roger.
After S4E6
Lake didn't feature directly, but the basement door may connect. The suicide-mission framing implies the same destination.
What's confirmed in canon
Pre-S1Mentioned in Ethan's children's books, which pre-existed his arrival.
S1Ethan dreams of the lake after the RV crash.
S4E3Jim's ghost (or what wears his likeness) tells Ethan to find it.
S4E5Life-sized dolls fished out come to life at nightfall. Talismans don't work. Roger killed.
Where the community lands
Two leading reads: dolls are vessels for the dead children; or the lake is the architect's site.
Still open
Q1Whose tears?
Q2Are the dolls the Anghkooey children?
Q3Did the original sacrifice happen here?
Q4Way out or trap inside trap?
Connected entities