This topic has been brought up a lot. I have 2 approaches to this.
Here is one.

proc/Pop()
spawn while(1) //loops
sleep(2000)//3 minutes 20 seconds
world.Repop()//built in proc that brings all destroyed mapped items back where they were mapped
world/New()//for when the world is created
..()
Pop()//calls the Pop proc

Or,

mob
Monster
var
startx
starty
startz
New()//when the mob is created
..()
src.startx=src.x
src.starty=src.y
src.startz=src.z
proc
DeathCheck()
if(src.health<=0)
new/mob/Monster(locate(src.startx,src.starty,src.startz))
del(src)