The Hidden Lake is a room for both members and non-members. It was first opened in the Cave Expedition where you needed to have the Amethyst Pin pin to come in here. It re-opened on March 15, 2010, where you didn't need any items to get in.
On the right side of the Hidden Lake, there is a door to the Underwater Room, which is open only to members. As of the Island Adventure Party 2010, there is a permanent entrance to the Hidden Lake from the Forest[1], making it not really "hidden" anymore.
the mine
The Hidden Lake
it is on the hidden lake in the lake down
In the mine
Hidden Lake.
in the hidden lake
Go to hidden lake.
Hidden lake
on top of hidden lake
at the hidden lake!
you dont
i just play