I first heard the story from our guide. Several students went inside the tunnel under this rock. Only one of them came out, and he came out from the Cathedral in Cuzco (used to be the Inca Palace). He also got a golden cup with him.
Other guides I met also mentioned about similar stories.
When I was inspecting the rock (it was close to sexy woman), I met a local woman. She speak to me in Spanish about the tunnels under the rock. I didn't understand a word. But I guess she said something about underground tunnels going from here to Pisac, Machu Picchu (highly unlikely, too far away), Cuzco etc . It collapsed or was sealed. Anyway I saw the entrance but couldn't go in to verify the story.