In "Welcome To The Temple: Part 1", when you get to the place with three rocks 
  stacked up on top of each other, take the left route. Follow it, then go down. 
  Destroy the crawler, and hit the wall on the side multiple times.
  In "Curse Of The Dead Ships: Part 1", continue along past the panel with 
  multiple mines. Keep going to the next one. At the bottom of the cave, release 
  Roadkill. He will find a secret passage leading down to two question marks.