The rock tunel is a natural tunnel found in the route 10, in Kanto. Even though it can look small at a first sight, there is a secret staircase leading to a much bigger dungeon below.
You are Pikachu, and are trapped inside.
You can run the game using:
./r.py
It needs python3 with curses support (so no Windows, afaik)
Copyright by Jaume Delclòs, aka cosarara97. The license is the WTFPL, included in the file COPYING