I thought I knew where they were, but I guessed wrong. Did anyone get it right?
I agree with you lgillette, that someone has been talking to Mildred. It's a little strange that she calls the kids 'now'.
They know where the treasure is, but someone is flooding the tunnels so they can't get out. Who could it be? The janitor, Professor Lowry, maybe even Mrs. Shegog?
I wonder if Mrs. Shegog's basement is where the tunnels end up and the kids are in there. They have found the treasure. Do you think someone grabbed Zach's phone before he could finish? Who turned the water on? Someone knows they're in the tunnels.