In the One Piece universe, the Calm Belt poses a significant obstacle to anyone attempting to travel across it. There are several factors that make crossing the Calm Belt a daunting and dangerous task:
Sea Kings: The Calm Belt is infested with gigantic sea creatures called Sea Kings, which are fearsome and powerful creatures that can easily sink ships. They are attracted to the vibrations caused by a ship's movement, making it nearly impossible for conventional vessels to pass through without being attacked.
Lack of Wind: The Calm Belt is notorious for its lack of wind, which makes it impossible for ships to rely on wind power to move forward. The absence of wind also prevents the use of conventional sailing techniques, further complicating the task of crossing the Calm Belt.
Lack of Log Pose Readings: The Calm Belt is devoid of magnetic fields, rendering Log Pose navigation unreliable. Log Poses are compass-like devices used to navigate the Grand Line, which rely on the presence of magnetic fields to guide ships. Without accurate Log Pose readings, it becomes extremely challenging to navigate through the Calm Belt.
Extreme Weather Conditions: The Calm Belt is known for its unpredictable and severe weather conditions. It is often hit by powerful storms, making it even more treacherous for ships attempting to cross.
Considering these challenges, attempting to fly over the Calm Belt would also be a difficult feat. The One Piece universe does not have widespread access to flight technology, and the few known flying vehicles are either extremely rare or limited in their capabilities. Furthermore, the vastness of the Grand Line, combined with its unpredictable nature and the challenges faced in navigating it, makes it highly improbable for anyone to reach the end of the Grand Line by simply flying over the Calm Belt.