Youth/Young Adult Book from Late 90s about a secret world that a boy (I think) discovered behind a brick wall in his city. The secret world had a pond, and a house over looking the pond.

[Sara A Pasquale]

I can't remember a lot of details, so I am just hoping that it rings a bell for someone. The book had a blue cover, and was either a youth or young adult novel. I read it in the late 90s. It was about a secret world that a boy (I think) discovered behind a brick wall in his city. The secret world had a pond, and a house over looking the pond. I remember that it was sort of strange, maybe a little bit of a thriller type book. I will try to remember more details, but any help would be great! Thanks.

[Kris]

Sara, what are the fantasy elements in this story?

What made it a thriller?

What's the real-world location (country, region, big city/small town)?

[Lobstergirl, au gratin]

Sara, are you still looking for this or did you find it?