Go to the The Explorer's Camp and go to the pile of rocks, begin to click the rocks, avoiding not to click the protector idol, as this will put the rocks in back in the way when you began, making you have to start all over again! When you are done, the upper rocks will reveal the Jade Skull, click it and you will obtain the Spy Eye. If you have already obtained the Spy Eye, you will be instead receive 500 coins.