Once inside, follow the path south until you come to a ledge with some worms on the bottom. Drop and keep heading south. Take the first turn west and keep heading west to the ???. (Note: The ledge you drop off is at H-9, head West for only a short time and take the first path South - if you see a room with tigers then you're in the right place. Keep right as much as possible to help avoid aggro.)
Run through into the middle of the room and approach the stairs ruins (G-10) for a very short cutscene with NPC Wawaroon.
The cutscene will start before you reach the base of the stairs.
>>WARNING!!<< The cutscene WILL drop your Invisible effect, the moment the cutscene ends the chigoes nearby might aggro you. They attack incredibly fast, and will kill you very quickly if unprepared. Recommend Blink and Stoneskin (if possible) before entering the room with the stairs.