This is book 15 of the "Rick Brant" series.

This note contains spoilers for the book.

In this one, Rick's sister Barbie and her friend Jan call them down to Virginia to investigate a ghost.

A blue ghost. Of a Confederate officer.

Which the girls almost believe in. Rick eventually proves it's being done with an 8mm film projector and dry ice. Being something of a showman, he does this by splicing some advertising material he has had custom made into the film loop that projects the ghost.

And, more important, he shows that it's being done to keep people away from the old lead mine (they often solve mysteries by figuring out what possible benefit could be conveyed to whom by the weird events). So they bring in the FBI to shut down the unranium mining scam being rum.

Along the way, Rick gets dropped 50 feet into a flooded quarry, and Rick and Scotty get trapped (deliberately) in the mine when the old rotten supports come down near the entrance. But this leads them to find the connection to the upper level, and the location of the projector (projecting out through an old sniper loophole).