The Hidden Lake is a subterranean lake in Club Penguin Rewritten. It opens out to the sea, and a variety of sunken treasure drifts here from underwater. Waterfalls flow here into the sea from underground reservoirs.
- At the Island Adventure Party: Rockhopper's Quest it was decorated with pink algae, rocks, and shells. A sign at the top of the room read "Mermaid Cove."
- A broken Aqua Grabber sub is seen in the bottom left corner.
- One instrument from the Instrument Hunt was also hidden here, in the broken Aqua Grabber sub.
- The key to the Underwater area, mainly called the Moss Key Pin, can be obtained by following the silhouette of the giant squid in Puffle Rescue to a reef with a secret entrance to the area itself.