You have to Morse Code SOS on the door. Do 3 fast taps on the door, then 3 times slower, then 3 fast taps. See related link for screenshots.
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You have to be a member. Go to the mine, and click on the 'Puffle Rescue' game. Click on the black puffle mission. When you see the squid, follow it, and stop yourself from drowning by going over the bubbles when they pop, and it should take you up. When you get to a maze of coral by following the squid, search around, and you'll find a key. Get it, and exit the game. Go to the hidden lake, and the door should now open.
All you have to do is press the door to open it! The purpose of this game is to figure out how to get the door open to get to the next level. For more help, see related links.
If you do not have keys for the doors in the game, you will not be able to open them. Even if you do have the keys, you have to use the key to open the door through your inventory menu. Once you close the doors, they are locked until you use the key to open it back up.
The button to exit your inventory is the same as the one you use to open it, by default it is the 'E' key.
In order to open the door you have to get to the other beach and there is something there that you get to open the door