What is the hardest question about submarines?

One of the hardest questions about submarines pertains to their operational safety and the limits of human endurance underwater. Factors like pressure, oxygen supply, and psychological stresses can significantly impact crew performance during extended missions. Additionally, understanding how submarines can remain undetected while executing complex maneuvers poses a significant challenge, intertwining technical prowess with strategic military considerations. Balancing these elements is crucial for the effective and safe operation of submarines.