under water appoxy or do the following
1. Take two rocks and place them as you wish
2. Mark both rocks in a place where they both touch fairly well.
3. Drill a small hole in both rocks in the spot you marked
4. In the bottom rock put a small amount of appoxy in the hole and insert a small fiberglass rod. it does not have to be fiberglass
5. Simply stick the other rocks hole onto the pin you just made.
6. Enjoy our new very sturdy rockwork.
Works perfectly, is 100% hidden, lets your move your rock, and also helps you make what would normaly be unstudry rock formations