Well, suppose we could try a solo thing, though that makes it intensely harder for me to be sneaky...
I figured you were gonna say that. What do you need from the backstory, honestly? The motive, means, and direction have already been decided.
Hmm... Explosives are loud, mine shafts are cold, this job seems horribly under-thought out (the pay or the blackmail must be incedible), and if they're all humans down there, why did they bother bringing him in at all? How is it supposed to be a secret entrance if every seperate team has to set off an explosion at the beginning of it? In no particular order.