Laydown delivery is a mode of attack using a freefall nuclear weapon: the bomb's descent to the target is slowed by ribbon parachute so that it actually lands on the ground before detonating. Laydown delivery requires the weapon's case to be reinforced so that it can survive the force of impact and generally involves a time-delay fuze to trigger detonation e.g. 45 seconds after hitting the ground. Laydown mode can be used to increase the effect of the weapon's blast on built-up targets such as submarine pens or to transmit a shock wave through the ground to attack deeply-buried targets. An attack of this type produces large amounts of radioactive fallout.