The Merman Chamber is a large circular room that has a big trident symbol on the floor, with 12 curved bioluminescent pillars surrounding it, and a stand with mysterious symbols on it. The roof above the trident symbol opens up every full moon to project the moonlight straight in from the night sky. It was built by mermen centuries ago, during the war against mermaids for ownership of Mako Island. Just like the Trident Chamber, the rock entrance to the cave only opens when exposed to full moonlight.
The Merman Chamber was found when Ondina and Mimmi wanted to test Zac's reaction to the moonlight. Zac and Mimmi saw the cave in a vision and traveled to Mako Island to look for it. Zac found it by feeling it's location. They opened the hidden entrance with Sirena's Moon Ring. Once Zac went near the stand and placed his hand over it, the symbols started to glow. Ondina tried to stop Zac, but was teleported out of the room into water somewhere. The next 2 full moons, Zac went to the chamber to activate its power but was interrupted both times.