Accessed via Ula’ula Meadow, this ancient ruin seems to have some connection to Alola’s Legendary Pokémon. If playing Ultra Sun, this place is known as the “Lake of the Moone”, while Ultra Moon players will know it as the “Lake of the Sunne”. Though the name may differ, it is basically the same place in both.
From the entrance, head directly north towards a stone bridge. To the left is a man who’s merely musing about the purpose of these ruins. Go across the bridge, then enter the giant doorway ahead. Inside the ruins, you’ll be in a wide hallway. Head over to the north-east corner and pick up TM03 Psyshock.
This move runs off the user’s Special Attack stat, but targets the opponent’s Defense stat. It can be useful for Psychic-types with low Attack, who struggle at dealing with Pokémon with high Special Defense. That’s actually everything you can do here. However, if you are keen to explore, proceed north towards the hallway exit.
In the next area, call Tauros and charge north all the way along the really, really, really long corridor. On the other side, you’ll emerge in front of a dirt path. At the end of the path is a flight of stairs leading up to an altar of sorts. You can look around and admire the scenery, but other than that, there’s nothing to do really do.
Once your curiosity has been sated, call Charizard and fly to the Ula’ula Meadow feather icon. You’ll be taken back to the north-east corner of the meadows, near the exit to the Lake of the Moone/Sunne. From here, you can return west to the top of the boardwalk. After that, continue north through the exit ahead to reach Route 17.