How do you get the Enigma Stone in Pokemon HeartGold and SoulSilver?

The Enigma Stone can be found on Three Island. First, use the Dowsing Machine to find a cave. Enter the cave and you'll find the Enigma Stone on the first floor. To reach the Enigma Stone, you'll need to use the Whirl Islands item. Be careful, though, as there are wild Pokemon in this cave!