Because it had a supposed connection to a fairy world.

"What do you think about a quick trip back into the fairy world?"

"Why, then you might have wandered too deep into the fairy world and been lost forever."

Once gone from the fairy world, they could never return.

We left the fairy world and claimed the lake country as our own.

No one ever went into the fairy world and came out again.

Journey far enough through it and you stand within the fairy world!

I told you that no one could go safely into the fairy world.

How you found your way clear of the fairy world, I will never know!

This is also a part of the green belt that runs through the fairy world.