I think he meant the underwater tunnel to Talpstar's Lair, past Montecito Hotel. To make it, go underwater with a bucket, build your tunnel as you want it but fill the inside with dirt first before adding the outer layer, then when it's done, go through it and remove the dirt.
I've had a look just now, it's looking good.
I'm still working on the bridge, though.
EDIT: I'm also thinking up how to make your suspended railways a little more aesthetically pleasing.