Make sure the only thing you have in your rucksack is a hammer. Go to Moonlight Cave (near the doctor's office). You will notice a hole in the ground somewhere. Go to it, and a message will appear asking if you would like to go in. Select "Yes". You will be at sub-level 2. You should see some bluish crystals and some black stones. Hit all of the crystals with X. If you find some gold, keep it. If you find some rare ore or stones, keep them. Fill your rucksack, then go to the stairs and exit the cave. Put all your finds into the Shipping Box. This trick works better with Lake Cave and if you only take rare ores and stones from there. Note: You will get low on energy and must find a way to restore it.