You can find Secret Treasures using Golden Shovels on the Big Land and on the Archipelago. Buy a Moneybox (Market >> tab with arrows), tap on it or on the icon with blue arrows in the bottom right corner, select the golden shovel icon and tap the objects or swipe through the field where you want to search. You will receive a pop-up message when you find one. Keep digging there to get items from rare Collections.