Go east from the Pokémon Center in Undella Town to reach Undella Bay.
You will need to use Flash and Strength in the Abyssal Ruins. If one of your Pokémon knows Dive, you can hold A while swimming on a dark circle in the water. When you are descending, you can hold Up to swim downward and hold Down to go back up. Keep going downward to find the Abyssal Ruins. Check back later for a map.