Why did Bushell want the prisoners to dig the tunnels so deep?

Bushell wanted the prisoners to dig the tunnels so deep to ensure they could escape undetected and avoid capture by the guards. A deeper tunnel would also provide a greater sense of security, as it would be less likely to be discovered during routine inspections. Additionally, digging deeper would allow for a more strategic exit point, increasing the chances of a successful escape. Overall, the depth of the tunnels was essential for the prisoners' safety and the effectiveness of their escape plan.