The Underwater Cave is an underground cavern that first appeared in "Gekko, Master of the Deep."
In the episode "Gekko, Master of the Deep," the PJ Masks sent Gekko to travel in the cave to stop Romeo whom he decided to use the cave as his hideout, while facing his fears.
In "Power Pondweed," the PJ Masks had to stop Romeo from obtaining the power pondweed that is powered up by the PJ Crystal Totem at the moat.
In "Master of the Moat," Romeo tricked the PJ Masks into thinking that Gekko would become "Master of the Moat" by using the Sky Crystal as bait.
In "Pondweed Party," Octobella tricked Orticia into helping her grow a pondweed plant to imprison the PJ Masks and trapped her with them so she can keep it growing in case she needed it again.