The Cave can be accessed once a week
Swimming back to the cave multiple times will make the message "You probably shouldn't disturb the cave too often..." appear on screen.
- Either eat Gillyweed or cast the Bubble-Head Charm on yourself or equip a Muggle Underwater Breathing Apparatus
- Enter the Main Yard
- Click the image of the lake in the top right hand corner of the Yard Image
- From "The Lake" click the text "Under the Lake" just below "Hogwarts - Main Yard
- From "Under The Lake" click "The Cave"
Swimming to the cave will either pop up a message "Something is inside the cave come back later" where or if the cave's empty click the "Approach" Button.
You will then feel beneath you and pull something up from the ground.