Explosions can be caused by explosive bloats, monsters with the explosive mutation, or by igniting swamp gas. Any creature caught in the explosion will take 15-20 damage or damage equal to half of their maximum health, whichever is higher, and is set on fire. Explosions reach for several (at least 3) squares beyond their source (the exploding monster or the edge of the gas cloud).
Submerged monsters will not be harmed by explosions. Armors of dampening will prevent the direct damage but will not prevent you from being set on fire. Being shielded will reduce the direct damage from the explosion.