Mazda 3 Owners Manual: Luggage Compartment
WARNING Do not place anything on top of the cover: Placing luggage or other cargo on top of the luggage compartment cover is dangerous. During sudden braking or a collision, the cargo could become a projectile that could hit and injure someone. The vehicle has a light weight luggage compartment cover to keep the contents of your luggage area out of sight; it will not retain heavy objects that are not tied down in an accident such as a rollover. Tie down all heavy objects, whether luggage or cargo, using the tie down hooks.
Make sure luggage and cargo are secured before driving: Not securing cargo while driving is dangerous as it could move or be crushed during sudden braking or a collision and cause injury.