Just read up on the various mine-signs in Discworld at http://wiki.lspace.org/wiki/Mine_sign. What is the Calling Dark and why is it regarded to be dangerous?
The only place the Calling Dark sign is mentioned is in the book Thud!,
"The Calling Dark, very dangerous."
That is all it says about this sign. We can infer from the preceding discussion about other Dark signs though,
Real like alive, sir,' said Carrot.Like they exist somewhere down in the dark under the world, and they cause themselves to be written. There's the Waiting Dark ... that's the dark that fills a new hole."
To me, the implication is that, if the Calling Dark is indeed a 'living' thing, it must be something that calls people into the darkness, drawing them away from the light, presumably to never be seen alive again.