There are many sinkholes in the cave, some of them will take you to new areas when you enter them. Other sinkholes will lead you back to the entrance of the cave. To save time, save the game and enter one of the sinkholes. If it leads you back to the entrance, restart the game to go back where you were.
At the entrance, go into one of the many sinkholes to find one that leads to a path of stairs being blocked off. In order to activate the stairs, you’ll need to activate two switches above it, which you need to go through many other sinkholes to even reach one of them (there is only one correct path of sinkholes that leads to one of them, save and restart if you go back to the entrance). After activating both of the switches, go back to find the stairs activated. Go up into a cave, which has many Garchomp statues that will chase and battle the player when near them.
After escaping through the exit, the player will enter an entirely new area, the Sugiline Ruins. Debris and many hidden items are scattered about the area. At the back, the player will find TM Stone Edge, a healing star, as well as an Arceus statue. Interacting with this statue will open up sinkholes that will lead you to the bottom floor of the ruins. Here, there are more items to pick up, such as a Medichamite. There are also many basins scattered about the area. Fill one with PokeSnax and leave by going into a sinkhole. Go back and return to the same spot to find a Gible. Gible will swim away if you approach it anywhere on its side, so you need to go to a sinkhole that has a basin directly under it to appear behind Gible and battle it.