The Titus Brothers Contractors company have won a government contract in Peru to blast a tunnel through a mountain and connect two isolated railroad lines. The deadline is approaching, and the contractors have hit a literal wall: excessively hard rock which defies conventional blasting techniques. The company is under pressure to finish, or else the contract will default to their rivals, Blakeson & Grinder. Mr. Job Titus has heard of Tom Swift and Tom's giant cannon, which is used in protecting the Panama Canal, and wants to hire Tom to develop a special blasting powder to help them finish the excavation.
Mr. Damon, Tom's very good friend, arrives in the middle of this conversation, and is unaware of the situation. By coincidence, Mr. Damon is invested in a business which procures cinchona bark from Peru, but production has all but ceased, prompting Mr. Damon to invite Tom to accompany him to Peru and discover the source of the problem.
Tom, Mr. Damon and Mr. Titus (along with Koku, Tom's giant) embark for Peru. On the way, they encounter Professor Swyington Bumper, who is on a lifelong quest to locate the lost city of Pelone. Professor Bumper returns to Peru each season, and has thus far been unsuccessful. When Professor Bumper discovers that Tom is headed to the same general area, Rimac, Professor Bumper decides to join the company.

Question: What problems occur when trying to build the tunnel?
Answer: They hit a wall of excessively hard rock
Alternative answer: They encounter excessively hard rock that can't be cut through with normal blasting techniques.