12 years later, I am having the exact same trouble and it's doing my head in.
There's a "shady wall" where I said the magic word to enter, then the first room inside is titled "underground". I don't know what location the "rock wall", but this driving me crazy!!!
EDIT: Ok, I figured it out too. So it turns out there are TWO entrances to the underground. One is on the mainland, near the towns. Its entrance is called "shady wall". But if you hope in the Gondola and go into the jungle, then head to near where you saved the Minx, there's a SECOND entrance. It's just called "rock wall". You have to use the mirrors at this second entrance.
Post edited October 16, 2023 by iiago