The Problem: Accessing the front of a boring machine that's already belowground is hard, particularly in deep tunnels, where the air pressure is dangerously high. Repairing cutting tools, therefore, is typically a task for workers who must spend time in hyperbaric chambers each time they visit the machine to acclimate to the pressure. (Two hundred feet belowground in an enclosed tunnel can get as high as 5 bars—the equivalent of being 165 feet deep in open water.) Crews retract the front of the machine to create a space ahead where a group of about five workers operates while wearing special helmets for breathing in those conditions. They use pneumatic wrenches and hammers to loosen the teeth, and pneumatic pulleys, hoists and chains to tug them out. After installing the new bits or disks, the team returns to the surface. Replacing a single tool could take up to four hours.