Minecraft wiki entry for Nether_portals, associated text: Portals built in the Overworld should be secured in order to prevent wandering monsters from entering the portal. These creatures can accumulate over time in the Nether; not only is there no sunlight to burn undead, but without a player present, time will barely pass for them (15 seconds for each new entry) so they may not have time to despawn. Mobs such as creepers are especially bad to handle in the Nether due to the soft nature of netherrack and the abundance of lava to fall into.