Instructions
Head to Undella Town via the Pokemon Map. Continue east once you are in the town until you reach Undella Bay.
Find one of the four areas of the bay that allow you to dive. These areas are marked by dark patches in the water. Click on the area with your stylus. Click ̶0;dive.̶1;
Dive into the water and you will immediately see the Abyssal Ruins. There four entrances to the Ruins, and each one goes to a different area of the Ruins. For example, the left most entrance goes to the first floor of the Ruins, while the second floor is accessed by the center entrance.