Laydown delivery is a mode of deploying a free-fall nuclear weapon in which the bomb's fall is slowed by parachute so that it actually lands on the ground before detonating. Laydown delivery requires that the weapon's case be reinforced so that it can survive the impact, and generally involves a time-delay fuse for detonation. Laydown mode can be used to increase the effect of the weapon's blast on built-up targets such as submarine pens.