Only members can get it because you can only get it on the black puffle level in puffle rescue. If you are a member then go on to this level and wait for the shadow. When it comes past you will see bubbles, you must jump on these to give you air. If you stay on one bubble too long it will pop and you will loose air (it took me ages to get across!) Soon you will come to this island and i think you will stairs. Go down them and you will come to a small cave. Lying near a huge shell you will see a key, pick it up. Now whenever you go to the hidden lake just click on the door in the pool and you will be in the secret cave!
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If you mean the one by the underwater cave you have to follow the squid on the first level of rescue the black puffle there is a squid follow it and you will come to a door which the key is hidden in.
You will get the key when you have rescued the black puffle in puffle rescue three times it does not work
key
You can buy a club penguin guide book. Then, look in the content for key merchandise and go to the page.
You go to the mine. You then play puffle rescue. Then play the black puffle version. Then you follow the octupus in the first level and find a secret room to get the black key and a key.