Dan recieves a letter to meet a man who is in charge of building a tunnel under the river. There is trouble with sabotage that will cause him to finish well behind schedule. A situation like that could ruin his reputation, get him fired, and it may be years before he could secure another contract.

Dan goes undercover into the tunnel to learn about the operation, and warned about the danger of getting the bends while moving from under the river back to the surface. Since Dan is a writer, his cover story is simple, he’s doing a story on Sand hogging, and writing about life in the tunnel.

As the story develops, we learn a little about the life and procedures involved in digging a tunnel. The men, equipment, dangers, and concerns for safety. Dan is making progress with getting in close with the work crew when a threat comes in. Dan urges the company owner to take it to the cops, but he’s too afraid of the negative publicity.

Now working as a sand hog, Dan gets a little too close to the saboteur for comfort and to get Dan out of the way, he is forced to the surface without proper decompression. It looks like the end for Dan Holiday when he is mistaken for being drunk, and the cops haul him in. Without immediate medical care, and being returned to a decompression chamber, he stands the real threat of dieing a painful death.

Dan’s secretary gets worried about him and convinces the cops to get him to medical help. There’s still a mystery to solve. Does Dan have enough clues to figure this one out? Who has been causing the trouble? Just as importantly, why are they doing it?